Negative electrode for non-aqueous electrolyte secondary cell and method for manufacture thereof, and non-aqueous electrolyte secondary cell

ABSTRACT

Disclosed is a negative electrode for a nonaqueous secondary battery included of a current collector and an active material structure containing an electro-conductive material having low capability of forming a lithium compound on at least one side of the current collector, the active material structure containing 5 to 80% by weight of active material particles containing a material having high capability of forming a lithium compound. The active material structure preferably has an active material layer containing the active material particles and a surface coating layer formed on the active material layer.

TECHNICAL FIELD

This invention relates to a negative electrode for nonaqueous secondarybatteries. More particularly, it relates to a negative electrode capableof intercalating and deintercalating a large amount of lithium andproviding a nonaqueous secondary battery with high energy density andimproved cycle life. The present invention also relates to a nonaqueoussecondary battery using the negative electrode.

BACKGROUND ART

Secondary batteries now used in mobile phones and notebook computers aremostly lithium ion secondary batteries, due to a higher energy densitythan other secondary batteries. With the latest tendency of mobilephones and personal computers toward multifunctionality, powerconsumption of these devices has shown a remarkable increase. Therefore,the demands for higher capacity secondary batteries have beenincreasing. As long as the present electrode active materials are used,it would be difficult to meet the increasing demands in the near future.

Lithium ion secondary batteries generally use graphite as a negativeelectrode active material. Now Sn alloys and Si alloys which offer 5 to10 times the capacity potential of graphite are being activelydeveloped. For instance, it has been proposed to produce Sn—Cu-basedalloy flakes by mechanical alloying, roll casting or gas atomization(see J. Electrochem. Soc., 148 (5), A471-A481 (2001)). Production ofNi—Si-based alloys and Co—Si-based alloys by gas atomization etc. isalso proposed (see JP-A-2001-297757). While these alloys have highcapacity, they have not yet been put to practical use on account of theproblems of large irreversible capacity and short cycle life.

There is an attempt to use copper foil, which is used as a currentcollector, electroplated with tin, as a negative electrode (seeJP-A-2001-68094). On the other hand, though silicon has higher capacitypotential than tin, there is no report on the development ofsilicon-containing plated copper foil for use in lithium ion secondarybatteries because silicon is an element incapable of electroplating.

The aforesaid Si alloys and Sn alloys and, in addition, Al alloys arenegative electrode active materials exhibiting high charge and dischargecapacities. However, they have the drawback that they incur largechanges in volume with alternate repetition of charging and dischargingand, as a result, undergo cracking and pulverizing and finally fall offthe current collector. To address this problem, techniques for preparinga negative electrode, of which the active material is prevented fromfalling off, have been proposed, in which a mixture of a negativeelectrode active material containing Si or an Si alloy and anelectro-conductive metal powder is applied to a conductive metal foil,followed by sintering in a non-oxidative atmosphere (see JP-A-11-339777,JP-A-2000-12089, JP-A-2001-254261, and JP-A-2002-260637). It has alsobeen proposed to prevent fall-off of a negative electrode activematerial by forming a thin film of Si on a current collector with goodadhesion by plasma-enhanced CVD or sputtering (see JP-A-2000-18499).Moreover, extensive studies have been devoted to development of variousSn— or Si-based intermetallic compounds (see JP-A-10-312804,JP-A-2001-243946, and JP-2001-307723). Even with these techniques,however, it is still impossible to perfectly prevent fall-off of thenegative electrode active material from the current collector as aresult of cracking and pulverizing of the active material, accompanyingcharge and discharge of a secondary battery.

JP-A-8-50922 proposes a negative electrode having a layer containing ametallic element capable of forming an alloy with lithium and a layer ofa metallic element incapable of forming an alloy with lithium. Accordingto the disclosure, this layer structure prevents the layer containingthe lithium alloy-forming metal element from cracking and pulverizingaccompanying charge and discharge of a battery. Judging from Examples ofthe publication, however, since the thickness of the layer of themetallic element incapable of forming a lithium alloy, which is theoutermost layer, is as extremely small as 50 nm, there is a possibilitythat the outermost layer fails to sufficiently coat the underlying layercontaining the lithium alloy-forming metal element. If so, the layercontaining the metallic element capable of forming a lithium alloycannot be sufficiently prevented from falling off when pulverized withrepeated charging and discharging of the battery. Conversely, if thelayer of the metal element incapable of forming a lithium alloycompletely covers the layer containing the lithium alloy-forming metalelement, the former layer would inhibit an electrolyte from passingthrough the latter layer, which will interfere with sufficient electrodereaction. No proposal has ever been made to satisfy these conflictingfunctions.

Besides the aforementioned, current collectors with appropriate surfaceroughness and current collectors having micropores that pierce thethickness are known to be used in lithium ion secondary batteries. Forexample, JP-A-8-236120 proposes a current collector formed of a porouselectrolytic metal foil having pores winding across the thickness andmaking a three-dimensional network. The porous electrolytic metal foilis produced by a process including the steps of electrodepositing ametal on the surface of a cathode drum to form an electrolytic foil ofthe metal and separating the foil from the drum, wherein an oxide filmhaving a thickness of at least 14 nm is formed on the surface of thecathode drum exposed after separation of the foil, and electrolyticmetal foil is deposited on the oxide film. The porosity and pore size ofthe metal foil are dependent on the thickness of the oxide film formedon the cathode drum. However, since the oxide film comes off little bylittle, together with the foil, it is difficult to control the porosityand pore size. Additionally, since the pores have a relatively smalldiameter and form a three-dimensional network, active material pasteapplied to one side of the foil and that applied to the other sidehardly come into contact with each other. There seems to be a limit,therefore, in improving the adhesion between the paste and the foil.

In order to solve the problems associated with the above-described metalfoil, Applicant previously proposed a porous copper foil formed byelectrodeposition such that copper grains, having an average planargrain size of 1 to 50 μm, are two-dimensionally bonded to one another.The porous copper foil has an optical transmittance of 0.01% or higherand a surface roughness difference of 5 to 20 μm, in terms of Rz,between the side in contact with a cathode for foil formation and theopposite side (see WO 00/15875). When the copper foil is used as acurrent collector of a lithium ion secondary battery, the followingadvantages are offered. (1) Since an electrolyte is able to pass throughthe copper foil so easily, even a limited amount of an electrolyte ispermitted to uniformly penetrate into an active material. (2) The copperfoil hardly interferes with donation and acceptance of Li ions andelectrons during charge and discharge. (3) Having proper surfaceroughness, the copper foil exhibits excellent adhesion to an activematerial. According to the process of making the porous copper foil,however, the electrolytic copper foil deposited on a cathode drum andseparated from the drum, is subjected to various processing treatments,which make the copper foil unstable. Therefore, the process cannot beseen as satisfactory in ease of handling the foil and fit for largevolume production. Additionally, a nonaqueous secondary battery using anegative electrode, prepared by applying a negative electrode activematerial mixture to the porous copper foil (a current collector), stillhas the problem that the negative electrode active material tends tofall off accompanying intercalation and deintercalation of lithium,resulting in reduction of cycle characteristics.

DISCLOSURE OF THE INVENTION

Accordingly, an object of the present invention is to provide a negativeelectrode for a nonaqueous secondary battery that can solve theaforementioned various problems and a nonaqueous secondary batteryhaving the negative electrode.

The present invention provides a negative electrode for a nonaqueoussecondary battery made up of an active material structure including anelectro-conductive material having low capability of forming a lithiumcompound on at least one side of a current collector. The activematerial structure contains 5 to 80% by weight of active materialparticles containing a material having high capability of forming alithium compound.

The present invention also provides a preferred process of producing thenegative electrode. The process comprises applying a slurry comprisingthe active material particles; an electro-conductive carbon material, abinder, and a diluting solvent to a surface of the current collector,drying the coating to form the active material layer, and electroplatingthe active material layer with the electro-conductive material havinglow capability of forming a lithium compound to form the surface coatinglayer.

The present invention also provides another preferred process ofproducing the negative electrode. The process comprises applying aslurry comprising the active material particles, an electro-conductivecarbon material, a binder, and a diluting solvent to a surface of thecurrent collector, drying the coating to form the active material layer,and depositing the electro-conductive material having low capability offorming a lithium compound on the active material layer by sputtering,chemical vapor deposition or physical vapor deposition to form thesurface coating layer.

The present invention also provides still another preferred process ofproducing the negative electrode. The process comprises forming a coatof a material different from the material making up the currentcollector on a carrier foil to a thickness of 0.001 to 1 μm,electroplating the carrier foil having the coat with the material makingup the current collector to form the current collector, applying aslurry comprising the active material particles, an electro-conductivecarbon material, a binder, and a diluting solvent to the surface of thecurrent collector, drying the coating to form the active material layer,electroplating the active material layer with the electro-conductivematerial having low capability of forming a lithium compound to form thesurface coating layer, and separating the current collector from thecarrier foil.

The present invention also provides a nonaqueous secondary batteryhaving the negative electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph showing the surface of a negativeelectrode according to the present invention.

FIG. 2 is an electron micrograph showing a cross-section of a negativeelectrode according to the present invention.

FIG. 3 is an electron micrograph showing a cross-section of anothernegative electrode according to the present invention.

FIG. 4 is an electron micrograph showing a cross-section of stillanother negative electrode according to the present invention.

FIG. 5(a), FIG. 5(b), FIG. 5(c), FIG. 5(d), FIG. 5(e), and FIG. 5(f)present a flow chart illustrating a process of preparing a porous metalfoil used as a current collector in a negative electrode of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described based on its preferredembodiments with reference to the accompanying drawings. FIG. 1 is anelectron micrograph taken of the surface of a negative electrodeaccording to an embodiment of the present invention. FIG. 2 shows anelectron micrograph taken of a cross-section of a negative electrodeaccording to the present invention. The negative electrode 1 has acurrent collector 2 having formed on one or both sides thereof an activematerial structure 5 containing an electro-conductive material havinglow capability of forming a lithium compound. The active materialstructure comprises active material particles containing a materialhaving high capability of forming a lithium compound. More specifically,the active material structure 5, which is formed on one or both sides ofthe current collector 2, has a layer 3 of active material particles(hereinafter referred to as an active material layer) and a surfacecoating layer 4 that is provided on the layer 3 as shown in FIG. 2.

The current collector 2 is made of a metal that can serve as a currentcollector of a nonaqueous secondary battery. It is preferably made of ametal that can serve as a current collector of a lithium secondarybattery. Such metals include copper, iron, cobalt, nickel, zinc, andsilver, and their alloys. Particularly preferred ones among them arecopper, a copper alloy, nickel or a nickel alloy. In using copper, thecurrent collector has the form of copper foil. Copper foil is obtainedby, for example, electrodeposition using a copper-containing solution. Apreferred copper foil thickness is 2 to 100 μm, still preferably 10 to30 μm. The copper foil obtained by the method described inJP-A-2000-90937 is particularly preferred because of its extremethinness with a thickness as small as 12 μm or less. Use of anelectrolytic metal foil as a current collector 2 is advantageous in thatthe adhesion between the current collector 2 and the active materiallayer 3 is improved because of the moderate surface roughness of anelectrolytic metal foil.

The active material layer 3 is a layer containing active materialparticles 7 which have a material having high capability of forming alithium compound. Such a material includes silicon materials, tinmaterials, aluminum materials, and germanium materials. The maximumparticle size of the active material particles 7 is preferably 50 μm orsmaller, still preferably 20 μm or smaller. The particle size,represented in terms of D₅₀ value, of the active material particles 7 ispreferably 0.1 to 8 μm, still preferably 0.3 to 1 μm. Where the maximumparticle size exceeds 50 μm, the active material particles 7 are liableto fall off, resulting in reduction of electrode life. The lower limitof the particle size is not particularly specified. The smaller, thebetter. In the light of the process of making the active materialparticles 7 (described later), the lower limit would be about 0.01 μm.The particle size of the active material particles 7 can be measured byMicrotrac, electron microscopic (SEM) observation. While it is desirablethat all the active material particles 7 fall under the recited particlesize range, it is no problem that greater active material particles 7are present in a small amount that does not impair the effects of theinvention.

It is preferred that voids be present in the active material layer 4.The voids serve to relax the stress which results from expansion andcontraction of the active material particles 7 due to intercalation anddeintercalation of lithium. In this connection, the proportion of thevoids in the active material layer 4 is preferably about 1 to 30% byvolume, still preferably about 5 to 30% by volume, particularlypreferably about 5 to 9% by volume. The proportion of the voids isobtained through mapping under an electron microscope. The proportion ofthe voids can be regulated within the recited range by forming theactive material layer by the process described later, followed bymechanically pressing the active material layer under appropriateconditions.

The active material layer 4 preferably contains an electro-conductivecarbon material in addition to the active material particles 7.Incorporation of the conductive carbon material adds improved electronconductivity to the active material structure 5. From this viewpoint,the amount of the conductive carbon material in the active materiallayer 3 is preferably 0.1 to 20% by weight, still preferably 1 to 10% byweight. To ensure the improvement on electron conductivity, it ispreferred for the electro-conductive carbon material to have the shapeof particles with a particle size of 40 μm or smaller, particularly 20μm or smaller. The lower limit of the particle size is not critical,which means the smaller, the better. In the light of the process ofmaking the particles, the lower limit would be about 0.01 μm. Theconductive carbon material includes acetylene black and graphite.

The surface coating layer 4 is a thick layer continuously covering thesurface of the active material layer 3, thereby the active materialparticles 7 are not substantially exposed. The surface coating layer 4generally covers the surface of the active material layer 3. The surfacecoating layer 4 has an almost uniform thickness, but some part 4 a ofthe surface coating layer 4 may enter into the active material layer 3.Some part of the surface coating layer 4 penetrating the active materiallayer 3 may reach the current collector 2. In some parts, the materialconstituting the surface coating layer 4 may penetrate the wholethickness of the active material layer 3 to reach the current collector.It is preferred that the material constituting the surface coating layer4 penetrate the active material layer 3 deeper and deeper therebyincreasing the electrical conductivity of the negative electrode as awhole. This is also preferred in that the penetrating materialconstituting the surface coating layer 4 forms a network structure andacts to prevent the active material particles 7 from falling off due toexpansion and contraction.

The active material particles 7 do not always need to be coveredcompletely with the surface coating layer 4, and part of them may beexposed. Taking into consideration, however, that the active materialparticles 7 should be prevented from falling off as a result ofpulverization due to intercalation and deintercalation of lithium, it isdesirable that the active material particles 7 be completely coveredwith the surface coating layer 4. Even though the active materialparticles 7 are completely covered with the surface coating layer 4, anelectrolyte and lithium are allowed to penetrate through micropores 6(described infra) into the inside of the surface coating layer 4 and toreact with the active material particles 7.

FIGS. 3 and 4 represent different examples of the negative electrode inwhich the active material layer 3 is completely covered with the surfacecoating layer 4. In FIGS. 3 and 4, the active material layer 3 formed onthe current collector 1 that is copper, contains silicon-copper alloyparticles, and the surface coating layer 4 that is copper is located onthe active material layer 3. The active material layer 3 is completelycovered with the surface coating layer 4. In the surface coating layer 4are observable fine breaks extending in the thickness direction. Voidsamong alloy particles are observable in the active material layer 3. InFIG. 3, it is seen that part of the surface coating layer 4 goes intothe active material layer 3 to such a degree that an alloy particle issurrounded with copper. In FIG. 4, on the other hand, the surfacecoating layer 4 is not so invasive into the active material layer 3, andthe two layers 3 and 4 are defined relatively clearly. Such a differencein layer geometry is ascribed to the process of producing the negativeelectrode.

The active material layer 3 being covered with the surface coating layer4, secondary batteries using the negative electrode of the presentinvention have an extended life compared with conventional ones. Evenwhen the active material particles 7 are pulverized due to intercalationand deintercalation of lithium, they maintain the electrical contactwith the surface coating layer 4 since they are shut away by the surfacecoating layer 4. As a result, the electron conductivity is maintained,and reduction in functions as a negative electrode is suppressed.Furthermore, the service life as a negative electrode can be prolonged.Where, in particular, part of the surface coating layer 4 enters intothe active material layer 3, the current collecting function is retainedmore effectively. If the active material is used as formed on thecurrent collector, it would be pulverized when intercalating anddeintercalating lithium and get isolated from the current collector. Itwould follow that the functions as a negative electrode reduce, and suchproblems as increase of irreversible capacity, reduction of charge anddischarge efficiency, and reduction of life will result.

The surface coating layer 4 is made of an electro-conductive materialhaving low capability of forming a lithium compound so as to beprevented from oxidation and fall-off Such conductive materials includecopper, silver, nickel, cobalt, chromium, iron, indium, and alloys ofthese metals (for example, copper-tin alloys). Of these metals preferredare copper, silver, nickel, chromium, cobalt, and alloys containingthese metals because of their particularly low capability of forming alithium compound. Electro-conductive plastics or electro-conductivepastes are also useful as a, conductive material. The expression “lowcapability of forming a lithium compound” as used herein means nocapability of forming an intermetallic compound or a solid solution withlithium or, if any, the capability is such that the resulting lithiumcompound contains only a trace amount of lithium or is very labile.

The surface coating layer 4 has on its surface a large number ofmicropores 6 that extend windingly in the thickness direction thereofSome of the numerous micropores 6 extend in the thickness direction ofthe surface coating layer 4 to reach the active material layer 3. Themicropores 6 are so small as having a width of about 0.1 μm to about 10μm when observed on a cut section of the surface coating layer 4. Smallas they are, the micropores 6 should have such a width as to allow anonaqueous electrolyte to penetrate. Be that as it may, a nonaqueouselectrolyte has a smaller surface tension than an aqueous one so that itis capable of penetrating sufficiently through the micropores 6 withsuch a small width.

When the surface coating layer 4 is observed from above through anelectron microscope, it is desirable for the micropores 6 to have anaverage opening area of 0.1 to 100 μm², preferably 1 to 30 μm². Withinthis range of opening area, the surface coating layer 4 effectivelyprevent the active material layer 3 from falling off while securingsufficient penetration of a nonaqueous electrolyte. For the same reason,it is preferred that the surface coating layer 4, when seen from above,have 1 to 30, still preferably 3 to 10, micropores 6 in every 100μm-side square in the visual field under an electron microscope. Thenumber of the micropores 6 as defined above is referred to as adistribution. For the same reason, when the surface coating layer 4 isobserved from above under an electron microscope, the ratio of the totalopening area of the micropores 6 in the visual field to the area of thevisual field (i.e., the open area ratio) is preferably 0.1 to 10%, stillpreferably 1 to 5%.

As can be seen from FIG. 1, the presence of the micropores 6 can beconfirmed through electron microscopic observation. In some cases,nevertheless, the micropores 6 are too tiny in their width to observeeven under an electron microscope. In such cases, the present inventionadapts the following method for confirming micropores 6. A negativeelectrode to be evaluated is assembled into a battery, and the batteryis subjected to one charge/discharge cycle. The cross-section of thenegative electrode is then observed with an electron microscope. If anychange in cross-sectional structure is observed between before and afterthe cycle, the negative electrode before the charge/discharge cycle isjudged to have had micropores 6. The grounds of this judgement are thatthe change of the cross-sectional structure, due to the charge/dischargecycle, is a result from the nonaqueous electrolyte's reaching the activematerial layer 3 through the micropores 6 distributed in the negativeelectrode before the charge and discharge and causing the lithium ionsof the nonaqueous electrolyte to react with the active materialparticles 7.

The micropores 6 allow a nonaqueous electrolyte to sufficientlypenetrate into the active material layer 3 and to sufficiently reactwith the active material particles 7. Fall-off of the active materialparticles 7, having pulverized due to charging and discharging, can beprevented by the thick surface coating layer 4 covering the surface ofthe active material layer 3. That is, since the active materialparticles 7 are shut up by the surface coating layer 4, fall-off of theactive material particles 7 attributed to lithium intercalation anddeintercalation can effectively be prevented. Generation of electricallyisolated active material particles 7 is effectively prevented andthereby the current collecting performance can be retained. As a result,reduction of functions as a negative electrode is suppressed. Extensionof the negative electrode life also results. In particular, where a part4 a of the surface coating layer 4 enters the active material layer 3,the current collecting function is retained more effectively. Asecondary battery using the negative electrode of the present inventionachieves a remarkably increased energy density per unit volume and unitweight over conventional ones and also enjoys a prolonged life.

The micropores 6 can be formed by various methods. For example, they canbe formed by mechanically pressing the surface coating layer 4 underproper conditions. A method of creating the micropores 6 in the surfacecoating layer 4 simultaneously with the formation of the surface coatinglayer 4 by electroplating as described later is especially preferred. Inmore detail, since the active material layer 3 contains the activematerial particles 7 as previously stated, it has a microscopicallytextured surface, that is, a mixed profile having active sites wheredeposit grows easily and sites where deposit does not grow easily. Whenthe active material layer 3 having such a surface condition iselectroplated, growth of the deposit differs from site to site, and theparticles of the material making up the surface coating layer 4 growinto a polycrystalline structure. On further growth of crystals,adjacent crystals meet, resulting in formation of voids in the meetingsite. The thus formed voids connect to each other to form the micropores6. According to this mechanism, there are formed micropores 6 having anextremely fine structure, and micropores 6 that extend in the thicknessdirection of the surface coating layer 4 can easily be created. Notinvolving outer force application, such as pressing force, to thesurface coating layer 4, the method is advantageous in that the surfacecoating layer 4 is not damaged, which means that the negative electrode1 is not damaged.

In order to effectively prevent fall-off of the active materialparticles 7 and to sufficiently maintain the current collectingfunction, it is preferred for the surface coating layer 4 to have alarge thickness of 0.3 to 50 μm, still preferably 0.3 to 10 μm,particularly preferably 1 to 10 μm. Even with such a large thickness,penetration of a nonaqueous electrolyte through the surface coatinglayer 4 is assured by the presence of the micropores 6. For securingsufficient negative electrode capacity, the thickness of the activematerial layer 3 is preferably 1 to 100 μm, still preferably 3 to 40 μm.The thickness of the active material structure 5 inclusive of thesurface coating layer 4 and the active material layer 3 is preferablyabout 2 to 100 μm, still preferably about 2 to 50 μm. The totalthickness of the negative electrode is preferably 2 to 200 μm, stillpreferably 10 to 100 μm, from the viewpoint of compactness and higherenergy density of the battery.

The amount of the active material particles 7 in the active materialstructure 5 inclusive of the active material layer 3 and the surfacecoating layer 4 is 5% to 80% by weight, preferably 10% to 50% by weight,still preferably 20% to 50% by weight. It is difficult to sufficientlyimprove the energy density of the battery with less than 5% by weight ofthe active material particles 7. More than 80% by weight of the activematerial particles 7 easily tend to, suffer from fall-off, which canresult in increased irreversible capacity, reduced charge and dischargeefficiency, and reduced battery life.

The active material particles 7 include (a) particles of single siliconor single tin, (b) mixed particles containing at least silicon or tinand carbon, (c) mixed particles containing silicon or tin and a metal,(d) particles of a compound of silicon or tin and a metal, (e) mixedparticles containing particles of a compound of silicon or tin and ametal and metal particles, and (f) single silicon or single tinparticles coated with a metal. The particles (a) to (f) can be usedeither individually or as a combination of two or more kinds thereofCompared with the particles (a), use of the particles (b) to (f) isadvantageous in that cracking and pulverizing of the active materialparticles 7 due to intercalation and deintercalation of lithium issuppressed more. This advantage is particularly conspicuous in using theparticles (f). When silicon is chosen, use of the particles (b) to (f)is also advantageous in that poor electron conductivity of silicon,which is semiconductive, can be compensated for.

In particular, where the mixed particles (b) containing at least siliconand carbon are used as active material particles 7, improved cycle lifeand negative electrode capacity are obtained for the following reason.Carbon, especially graphite, which is used in a negative electrode ofnonaqueous secondary batteries, contributes to intercalation anddeintercalation of lithium, provides a negative electrode capacity ofabout 300 mAh/g, and is additionally characterized by its very smallvolumetric expansion on lithium storage. Silicon, on the other hand, ischaracterized by as high a negative electrode capacity as about 4200mAh/g, 10 times or more the negative electrode capacity of graphite.Nevertheless, volumetric expansion of silicon on lithium storage reachesabout 4 times that of graphite. Then, silicon and carbon such asgraphite are mixed at a predetermined ratio and ground by, for example,mechanical milling to obtain uniformly mixed powder having a particlesize of about 0.1 to 1 μm. When this mixed powder is used as an activematerial, the volumetric expansion of silicon on lithium storage isrelaxed by graphite to provide improved cycle life, and a negativeelectrode capacity ranging about 1000 to 3000 mAh/g is obtained. Theamount of silicon in the mixed powder is preferably 10 to 90% by weight.The amount of carbon in the mixed particles is preferably 10 to 90% byweight. Increased battery capacity and extended negative electrode lifewill be secured with the mixed particles composition falling within theabove range. Moreover, no compound such as silicon carbide is formed inthe mixed particles.

The mixed particles (b) as active material particles 7 may be amulti-component mixture containing other metal element(s) in addition tosilicon or tin and carbon. The other metal element is at least oneelement selected from the group consisting of Cu, Ag, Li, Ni, Co, Fe,Cr, Zn, B, Al, Ge, In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd.

In using the mixed particles (c) of silicon or tin and a metal as activematerial particles 7, the metal in the mixed particles (c) includes atleast one of Cu, Ag, Li, Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn (except forcases where the particles 7 contain tin), Si (except for cases where theparticles 7 contain silicon), In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr,Pd, and Nd. Preferred of these metals are Cu, Ag, Ni, Co, and Ce. It isparticularly desirable to use Cu, Ag or Ni for their excellent electronconductivity and low capability of forming a lithium compound. Use of Lias the metal is also preferred. In that case, the active materialcontains metallic lithium from the beginning, which produces advantages,such as reduction of irreversible capacity, improvement oncharge/discharge efficiency, and reduction in volumetric change leadingto improved cycle life. In the mixed particles (c), the amount ofsilicon or tin is preferably 30% to 99.9% by weight, still preferably50% to 95% by weight, particularly preferably 75% to 95% by weight. Theamount of the metal, such as copper, is preferably 0.1% to 70% byweight, still preferably 5% to 50% by weight, particularly preferably 5%to 30% by weight. Increased battery capacity and extended negativeelectrode life will be secured with the mixed particles compositionfalling within the above range.

The mixed particles (c) can be prepared, for example, as follows.Silicon particles or tin particles and metal particles, such as copperparticles, are mixed and pulverized simultaneously by use of apulverizer, which includes an attritor, a jet mill, a cyclon mill, apaint shaker, and a fine mill. Pulverization in these pulverizers may beeither in a dry system or a wet system. Wet pulverization is preferredfor particle size reduction. The particle size before pulverization ispreferably about 20 to 500 μm. Mixing and pulverizing in a pulverizerresult in formation of uniformly mixed powder of silicon or tin and themetal. The particle size of the resulting powder can be adjusted to,e.g., 40 μm or smaller by properly controlling the operation conditionsof the pulverizer. Thus are prepared the mixed particles (c).

Where the active material particles 7 are (d) particles of a compound ofsilicon or tin and a metal, the compound includes an alloy of silicon ortin and a metal, which is any one of (i) a solid solution of silicon ortin and the metal, (ii) an intermetallic compound of silicon or tin andthe metal, and (iii) a composite having at least two phases selectedfrom a single phase of silicon or tin, a single phase of the-metal, asolid solution of silicon or tin and the metal, and an intermetalliccompound of silicon or tin and the metal. The metal can be selected fromthose recited above as the metal used in the mixed particles (c).Similarly to the mixed particles (c), the silicon or tin/metal compoundparticles preferably comprise 30% to 99.9% by weight of silicon or tinand 0.1% to 70% by weight of the metal. A still preferred composition ofthe compound is selected appropriately according to the process ofproducing the compound particles. For instance, where the compound is asilicon or tin/metal binary alloy prepared by a quenching processdescribed infra, a preferred amount of silicon or tin is 40% to 90% byweight, and a preferred amount of the metal, e.g., copper, is 10% to 60%by weight.

Where the compound is a ternary or higher order alloy containing siliconor tin and metals, the above-described binary alloy further contains asmall amount of an element selected from the group consisting of B, Al,Ni, Co, Fe, Cr, Zn, In, V, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd.Such an additional component produces an additional effect in preventingcracking and pulverizing of the active material particles. To enhancethe effect, a preferred amount of the additional component in thesilicon or tin/metal alloy is 0.01% to 10% by weight, particularly 0.05%to 1.0% by weight.

Where the compound particles (d) are alloy particles, the alloyparticles are preferably prepared by a quenching process hereinafterdescribed. The quenching process is advantageous in that the resultingalloy crystallites have a small size and are uniformly dispersible toprovide an active material layer that will be prevented from crackingand pulverizing and maintain electron conductivity. The quenchingprocess starts with preparing a molten metal of raw materials includingsilicon or tin and a metal, e.g., copper by high frequency melting. Theratio of silicon or tin and the metal in the molten metal is selectedfrom the above-specified range. The molten metal temperature ispreferably 1200° to 1500° C., still preferably 1300° to 1450° C., inconnection to the quenching conditions. An alloy is made from the moltenmetal by mold casting. That is, the molten metal is poured into acopper- or iron-made mold and quenched to obtain an alloy ingot, whichis ground and sieved to obtain particles, e.g., of 40 μm or smaller foruse in the present invention. A roll casting process can be used insteadof the mold casting process. In a roll casting process, the molten metalis injected onto the peripheral surface of a roll which is made ofcopper and rotates at a high speed. For quenching the molten metal, therotating speed of the roll is preferably 500 to 4000 μm, stillpreferably 1000 to 2000 rpm. The rotating speed in terms of peripheralspeed is preferably 8 to 70 m/sec, particularly preferably 15 to 30m/sec. When the molten metal having the above-specified temperature isquenched on the roll rotating at the above-specified speed, the coolingrate reaches 10² K/sec or higher, preferably 10³K/sec or higher. Theinjected molten metal is rapidly cooled on the roll and made into a thinstrip, which is ground and sieved to obtain particles having a particlesize, e.g., of 40 μm or smaller for use in the present invention.Particles of desired size can also be prepared by a gas atomizationprocess instead of the quenching process. In a gas atomization process,a jet of an inert gas such as argon is applied to the molten metal at1200° to 1500° C. under a gas pressure of 5 to 100 atm to atomize andquench the molten metal. An arc melting process or mechanical millingcan also be used.

Where the active material particles are the mixed particles (e)containing particles of a compound of silicon or tin and a metal andmetal particles, the compound particles described with respect to theparticles (d) and the metal particles described with respect to themixed particles (c) can be used in the mixed particles (e). The metalelement contained in the compound particles and the metal element of themetal particles may be either the same or different. In particular, whenthe metal element of the compound particles is nickel, copper, silver oriron, and the metal element of the metal particles is nickel, copper,silver or iron, these metals easily form a network structure in theactive material layer 3. Such a metal network structure is effective inimproving the electron conductivity and preventing fall-off of theactive material particles 7 due to expansion and contraction. Takingthese effects into consideration, it is preferred that the metal elementin the compound particles and that of the metal particles be the same.The active material particles (e) are obtained by first preparingcompound particles in the same manner as for those of the particles (d)and then mixing the compound particles with metal particles in the samemanner as for the production of the mixed particles (c). The silicon ortin to metal ratio in the compound particles can be the same as in thecompound particles (d). The compound particles to metal particles ratiocan be the same as the ratio of silicon or tin particles to metalparticles in the mixed particles (c). With respect to other particularsof the active material particles (e), the description given to the mixedparticles (c) and the compound particles (d) apply appropriately.

Where the active material particles 7 are (f) the single silicon orsingle tin particles coated with a metal (hereinafter referred to as“metal-coated particles”), the coating metal is selected from theabove-recited metals used in the particles (c) and (d), for example,copper (except Li). The amount of silicon or tin in the metal-coatedparticles is preferably 70% to 99.9% by weight, still preferably 80% to99% by weight, particularly preferably 85 to 95. The amount of thecoating metal, such as copper, is preferably 0.1% to 30% by weight,still preferably 1% to 20% by weight, particularly preferably 5% to 15%by weight. The metal-coated particles can be prepared by, for example,electroless plating. In carrying out the electroless plating, a platingbath having silicon particles or tin particles suspended therein andcontaining a coating metal (e.g., copper) is prepared. The siliconparticles or tin particles are electroless plated in the plating bath todeposit the coating metal on the surface of the silicon particles or tinparticles. A preferred concentration of the silicon particles or tinparticles in the plating bath is about 400 to 600 g/l. In electrolessplating using copper as a coating metal, the plating bath preferablycontains copper sulfate, Rochelle salt, etc. A preferred concentrationof copper sulfate and that of Rochelle salt are 6 to 9 g/l and 70 to 90g/l, respectively, from the viewpoint of plating rate control. From thesame viewpoint, the plating bath preferably has a pH of 12 to 13 and atemperature of 20 to 30° C. The plating bath contains a reducing agent,such as formaldehyde, in a concentration of about 15 to 30 cc/l.

Where the active material particles 7 are silicon-containing particles,it is preferred for the particles to have an average particle size (D₅₀)of 0.1 to 10 μm, particularly 0.3 to 8 μm, especially 0.8 to 5 μm,whichever of the forms (a) to (e) the silicon-containing particles mayhave. In other words, the silicon-containing active material particlesare preferably fine particles with a small diameter (hereinafterreferred to as “small-diametered active material particles”). Use ofsuch small-diametered active material particles results in reducedfall-off of the active material particles from the negative electrodeand makes it feasible to extend the life of the negative electrode. Inmore detail, active material particles will change greatly in volumeupon intercalating and deintercalating lithium and are to bedisintegrated into microcrystallites or finer particles in due course oftime. It follows that cracks develop, and part of the active materialparticles lose electrochemical contact among themselves, which causesreduction in charge/discharge cycle characteristics important for asecondary battery. For this reason, fine particles of small size areused in the negative electrode from the very beginning thereby tosuppress further size reduction of the particles during charging anddischarging and to improve the charge/discharge cycle characteristics.Incidentally, if the small-diametered active material particles have anaverage particle size smaller than the lower limit of theabove-specified range, the particles are susceptible to oxidation.Moreover, such small particles are costly to produce. The particle sizeof the small-diametered active material particles is measured by a laserdiffraction scattering method or under electron microscopic (SEM)observation.

Having a large surface area, small-diametered active material particlesare more susceptible to oxidation than relatively large-diameteredparticles (e.g., those having a diameter of several tens ofmicrometers). Oxidation of active material particles causes increase ofirreversible capacity and reduction of charge/discharge efficiency.Irreversible capacity and charge/discharge current efficiency areimportant characteristics for secondary batteries similarly to thecharge/discharge cycle characteristics. In some detail, if much oxygenis present in small-diametered active material particles,electrochemically intercalated lithium ions form firm bonding withoxygen atoms. It would follow that the lithium ions are not released indischarging. Accordingly, small-diametered active material particlesneed stricter control of oxygen concentration than relativelylarge-diametered particles. Specifically, the concentration of oxygenpresent in the small-diametered active material particles is preferablyless than 2.5% by weight, still preferably 1.5% by weight or lower,particularly preferably 1% by weight or lower. In contrast, relativelylarge-diametered particles, whose surface area is not so large, do notrequire such severe control against oxidation. It is desirable for thesmall-diametered active material particles to have as low an oxygenconcentration as possible. It is most desirable that no oxygen bepresent. In the light of the process of producing the small-diameteredactive material particles, nevertheless, a presently reachable lowestoxygen concentration would be about 0.005% by weight. The oxygenconcentration in small-diametered active material particles is measuredby gas analysis involving combustion of a sample to be analyzed.

In addition to the preferred oxygen concentration of the wholesmall-diametered active material particles, it is also preferred thatthe Si concentration in the outermost surface of the small-diameteredactive material particles be higher than ½, still preferably not lessthan ⅘, particularly preferably not less than 10 times, the oxygenconcentration in the outermost surface of the particles. As a result ofinvestigations the present inventors have revealed that an increase ofirreversible capacity and a decrease of charge/discharge currentefficiency are affected predominantly by the oxygen concentration of theoutermost surface of the small-diametered active material particles.This is because the oxygen present in the outermost surface easilyundergoes reaction with lithium during charging of the secondarybattery, which can deteriorate the battery characteristics. Hence, theSi concentration to oxygen concentration ratio in the outermost surfaceof the particles is specified as described above. The surface oxygenconcentration of small-diametered active material particles can bemeasured with various surface analyzers including an electronspectroscope for chemical analysis (ESCA) and an Auger electronspectroscope (AES).

Whichever of the particles (a) to (e) may be used, the small-diameteredactive material particles are preferably produced under conditionsinhibiting contamination of oxygen, for example, in an inert gasatmosphere.

Whichever of the particles (a) to (e) may be used, the small-diameteredactive material particles are pulverized to an average particle sizewithin the above-recited range by a prescribed pulverization process,typically exemplified by a dry pulverization process and a wetpulverization process. In dry pulverization, a jet mill is used, forexample. In wet pulverization, the particles are dispersed in an organicsolvent (grinding liquid), such as hexane or acetone, and groundtogether with a grinding medium, such as alumina beads or zirconiabeads.

During the pulverization operation, the small-diametered active materialparticles are often oxidized. It is therefore preferred that thepulverized small-diametered active material particles, the averageparticle size D₅₀ of which has been reduced to 0.1 to 10 μm, besubjected to etching with an etching solution to remove the oxide on thesurface of the particles. By so doing, the oxygen concentration of thewhole small-diametered active material particles and the oxygenconcentration of the outermost surface of the particles can easily becontrolled to or below the recited values. Useful etching solutionsinclude aqueous solutions of HF, buffered acids, NH₄F, KOH, NaOH,ammonia or hydrazine. The degree of etching can be controlledappropriately by the kind and concentration of the etching solution, thetemperature of the etching solution, the etching time, and the like. Asa result, the oxygen concentration of the whole small-diametered activematerial particles and the oxygen concentration of the outermost surfaceof the particles can easily be controlled within the recited ranges.Note that, however, the oxide on the particle surface should not beremoved completely in the etching step. This is because particles fromwhich the surface oxide has completely been removed would be oxidizedrapidly when they are exposed to the atmosphere. Therefore, the degreeof etching is preferably controlled so that an adequate amount of theoxide may remain. Even after being exposed to the atmosphere, thoseparticles having an adequate amount of the oxide remaining on thesurface thereof are capable of maintaining almost the same surface andwhole oxygen concentrations as adjusted by the etching.

When etching is effected using HF, for example, the small-diameteredactive material particles are put into an HF solution having aconcentration of about 1 to 50% by weight, and the system is stirred atroom temperature for about 5 to 30 minutes, whereby the surface oxygenconcentration can be reduced to a desired level. When in using KOH orNaOH for etching, the small-diametered active material particles are putinto an aqueous solution having a concentration of about 1 to 40% byweight, and the system is stirred at room temperature for about 5 to 120minutes. In using ammonia, the small-diametered active materialparticles are put into an aqueous solution having a concentration ofabout 1 to 20% by weight, and the system is stirred at room temperaturefor about 5 to 60 minutes to carry out etching. When NH₄F is used, thesmall-diametered active material particles are put into an aqueoussolution having a concentration of about 1 to 50% by weight, followed bystirring at room temperature for about 5 to 60 minutes to conductetching. In using hydrazine, the small-diametered active materialparticles are put into an aqueous solution having a concentration ofabout 1 to 50% by weight, followed by stirring at room temperature forabout 5 to 60 minutes to compete etching.

The negative electrode containing the above-described small-diameteredactive material particles is less susceptible to cracking andpulverizing with repetition of charge/discharge cycles. As a result,charge/discharge efficiency increases, and irreversible capacity reducesthereby to improve the charge/discharge cycle characteristics.Furthermore, reduction in oxygen content in the small-diametered activematerial particles also brings about reduction of irreversible capacity,increase of charge/discharge efficiency, and improvement incharge/discharge cycle characteristics.

The small-diametered active material particles may be coated with a thinmetal coat. The thin metal coat inhibits oxidation of thesmall-diametered active material particles to effectively prevent anincrease in irreversible capacity and a decrease in charge/dischargecurrent efficiency. In addition, the electron conductivity is improved,and the charge/discharge cycle characteristics are further improved.

In order to inhibit oxidation of the small-diametered active materialparticles more effectively and to allow Li and Si to react with eachother more efficiently, the thickness of the thin metal coat ispreferably 0.005 to 4 μm, still preferably 0.05 to 0.5 μm. The thicknessof the thin metal coat is measured with, for example, ESCA or AES.

The metal making up the thin metal coat is preferably selected fromthose having low capability of forming lithium. Such metals include Ni,Cu, Co, Fe, Ag, and Au Ni, Co, Ag, and Au are still preferred from thestandpoint of oxidation prevention. These metals can be used eitherindividually or in the form of an alloy composed of two or more thereof.

In the small-diametered active material particle coated with a thinmetal coat, the oxygen concentration in the interfacial part between thethin metal coat and the small-diametered active material particle issuch that the Si concentration exceeds ½ the oxygen concentration asdescribed with reference to the aforementioned small-diametered activematerial particles. The “interfacial part” between the thin metal coatand the small-diametered active material particle is considered to bethe part where the concentration of the metal making up the thin metalcoat becomes the minimum in AES analysis of the metal-coatedsmall-diametered active material particles.

The lower the oxygen concentration of the outermost surface of the thinmetal coat, the more desirable for increasing the electricalconductivity of the metal-coated small-diametered active materialparticles.

The small-diametered active material particles having a thin metal coatare preferably prepared as follows. Active material particles arepulverized to powder of prescribed size in a dry or wet process inaccordance with the above-described process for preparingsmall-diametered active material particles. The oxide present on thesurface of the particles is removed by etching. The etched particles arethoroughly rinsed with water and then subjected to electroless platingto form a thin metal film thereon. Prior to the electroless plating, theparticles may be subjected to a surface sensitizing treatment and asurface activating treatment in a usual manner. The electroless platingconditions are selected appropriately according to the plating metal.For instance, the plating bath composition shown below is useful for Niplating. In this case, the bath has a temperature of about 40 to 60° C.and a pH of about 4 to 6, and the plating time is 0.5 to 50 minutes.NiSO₄.6H₂O 15-35 g/l NaH₂PO₂.H₂O 10-30 g/l Na₃C₆H₅O₇ 15-35 g/l NaC₃H₅O₂ 5-15 g/l

The thin metal coat formed on the small-diametered active materialparticles does not always need to cover the individual particlescompletely. For example, the thin metal coat covering the whole particleuniformly may have a large number of micropores extending through thethickness thereof. Such micropores allow an electrolyte to pass throughand reach the inside of the small-diametered active material particle,so that the electrochemical reactivity essentially possessed by thesilicon-containing particle may surely be manifested. The thin metalcoat may also be provided in the form of islands on the particlesurface.

A preferred process for producing the negative electrode of the presentinvention will be described. This process starts with preparation of aslurry to be applied to the surface of a current collector. The slurrycomprises active material particles, electro-conductive carbon materialparticles, a binder, and a diluting solvent. The active materialparticles and the electro-conductive carbon materials have previouslybeen described. The binder that can be used includes polyvinylidenefluoride (PVDF), polyethylene (PE), and ethylene-propylene-diene monomer(EPDM). The diluting solvent includes N-methylpyrrolidone andcyclohexane.

The amount of the active material particles in the slurry is preferablyabout 14% to 40% by weight. The amount of the electro-conductive carbonmaterial is preferably about 0.4% to 4% by weight. The amount of thebinder is preferably about 0.4% to 4% by weight. The amount of thediluting solvent is preferably about 60% to 85% by weight.

The slurry is applied to the surface of a current collector. The currentcollector may be prepared separately or in the same line for producingthe negative electrode of the invention. In the latter case, the currentcollector is preferably prepared by electrodeposition. The spread of theslurry on the current collector is preferably such that the drythickness of the active material layer is about one to three times thethickness of a finally obtained active material structure. After thecoating of the slurry dries to form an active material layer, thecurrent collector having the active material layer formed thereon isimmersed in a plating bath containing an electro-conductive materialhaving low capability of forming a lithium compound and electroplated inthis state with the conductive material to form a surface coating layeron the active material layer. By using this process, a surface coatinglayer with a large number of micropores can easily be formed. In moredetail, since the active material layer 3 has a microscopically texturedsurface as described supra, there are active sites where deposit growseasily and sites where deposit does not grow easily in a mixed state.When the active material layer 3 having such a surface condition iselectroplated, the deposit grows non-uniformly, and the particles of thematerial making up the surface coating layer 4 grow into apolycrystalline structure. On further growth of crystals, adjacentcrystals meet, resulting in formation of microvoids in the meeting site.The following is recommended electroplating conditions taking copper,for instance, as an electro-conductive material. In using a coppersulfate-based solution, electroplating is performed at a copperconcentration of 30 to 100 g/l, a sulfuric acid concentration of 50 to200 g/l, a chlorine concentration of 30 ppm or lower, a bath temperatureof 30 to 80° C., and a current density of 1 to 100 A/dm². Under theseelectrolysis conditions, it is easy to form a surface coating layer partof which enters into the active material layer or penetrates and reachesthe current collector or a surface coating layer penetrating throughoutthe active material layer. In another electrolysis system, a copperpyrophosphate-based solution can be used. In this case, electroplatingis conducted at a copper concentration of 2 to 50 g/l, a potassiumpyrophosphate concentration of 100 to 700 g/l, a bath temperature of 30to 60° C., a pH of 8 to 12, and a current density of 1 to 10 A/dm².

After the surface coating layer is thus formed on the active materiallayer, the active material layer as covered with the surface coatinglayer may be subjected to mechanical pressing thereby densify the activematerial layer. As a result of densification, the voids among the activematerial particles and the conductive carbon material particles arefilled with the conductive material constituting the surface coatinglayer to make a structure in which the active material particles and theconductive carbon material particles are dispersed. Furthermore, theseparticles and the surface coating layer come into closer contact toimprove electron conductivity. Additionally, the void volume of theactive material layer is adjusted appropriately to relax the stressresulting from the active material particles' expansion and contractiondue to intercalation and deintercalation of lithium. In order to obtainsufficient electron conductivity, the densification by mechanicalpressing is preferably such that the total thickness of the activematerial layer and the surface coating layer after mechanical pressingmay be 90% or less, particularly 80% or less, of that before mechanicalpressing. Mechanical pressing can be carried out with, for example, aroll press.

In this process of production, it is possible to mechanically press theactive material layer before the electroplating. For the sake ofdistinguishing from the above-mentioned mechanical pressing, themechanical pressing before the electroplating will be calledprepressing. Prepressing is effective in preventing separation betweenthe active material layer and the current collector and preventing theactive material particles from being exposed on the surface coatinglayer. As a result, deterioration of battery cycle life due to fall-offof the active material particles can be averted. The prepressingconditions are preferably such that the thickness of the active materiallayer after prepressing is 95% or less, particularly 90% or less, ofthat before prepressing.

The electroplating used in the process for forming the surface coatinglayer may be replaced with sputtering, chemical vapor deposition orphysical vapor deposition. The surface coating layer may also be formedby rolling an electro-conductive foil, for example, by rolling a metalfoil, a metal mesh foil or an electro-conductive plastic film. In usingthese materials, creation of micropores in the surface coating layer canbe achieved by pressing under so controlled conditions.

In another preferred process of producing the negative electrode of thepresent invention, a dispersion plating technique is employed. Toconduct dispersion plating, a plating bath having suspended thereinactive material particles and containing an electro-conductive materialhaving low capability of forming a lithium compound is prepared. Inorder to incorporate a sufficient amount of the active materialparticles into the active material structure, the amount of the activematerial particles in the plating bath is preferably 200 to 600 g/l,still preferably 400 to 600 g/l. In using copper as anelectro-conductive material having low capability of forming a lithiumcompound and copper sulfate as a copper source, the plating bathpreferably has the following formulation in view of plating ratecontrollability and capability of building up a surface coating layer toa thickness enough to sufficiently hold an active material layer of theactive material particles. A preferred copper concentration is 30 to 100g/l. A preferred sulfuric acid concentration is 50 to 200 g/l. Apreferred chlorine concentration is 300 ppm or lower. A preferredcresolsulfonic acid concentration is 40 to 100 g/l. A preferred gelatinconcentration is 1 to 3 g/l. A preferred β-naphthol concentration is 0.5to 2 g/l.

A current collector is immersed in the plating bath, and electroplatingis started in this state. The current density used in the electrolysisis preferably about 1 to 15 A/dm² from the standpoint of plating ratecontrol. The plating bath temperature is room temperature, which isaround 20° C. By this electroplating, the metal in the plating bath isreduced to form a surface coating layer and, at the same time, an activematerial layer covered with the surface coating layer is formed on thesurface of the current collector. In order to form the active materiallayer uniformly, the electrolysis may be effected while stirring theplating bath.

While the current collectors that can be used in the present inventionhave been described supra, a current collector formed of the followingporous metal foil is also preferably used. The porous metal foil(hereinafter simply called a metal foil) has a great number ofmicropores. It has both micropores piercing therethrough in thethickness direction thereof and micropores that are closed within thethickness thereof The term “micropores” as used herein is intended toindicate those holes which pierce the foil in the thickness direction.This does not mean that a metal foil having micropores that are closedwithin the thickness of the foil is excluded, nor that such a metal foilis unfavorable.

The above-described metal foil, when used as a current collector of anonaqueous secondary battery, secures sufficient passagewaystherethrough for an electrolyte thereby bringing about a furtherincrease in battery capacity. Moreover, the active material is moreeffectively prevented from falling off from the electrode as a result ofintercalating and deintercalating lithium.

The micropores of the metal foil preferably have a diameter of 0.01 to200 μm, still preferably 0.05 to 50 μm, particularly preferably 0.1 to10 μm. Micropores with a diameter less than 0.01 μm can fail to securepassage of a nonaqueous electrolyte sufficiently. Where the porediameter exceeds 200 μm, the metal foil strength tends to reduce inrelation to the foil thickness described below, the active materialtends to fall off with intercalating and deintercalating of lithium, andthe resulting nonaqueous secondary battery tends to have reduced cyclecharacteristics. Not all the pores piercing the metal foil are requiredto have a diameter falling within the recited range. It is acceptablethat the metal foil has a very small number of micropores with diametersout of that range that are unavoidably created in the course of metalfoil manufacturing.

The number of micropores whose diameter is in the recited range per unitarea (pore density) is preferably 5 to 10000/cm², still preferably 10 to5000/cm², particularly preferably 100 to 2000/cm², in every part of themetal foil. Metal foil with a pore density of less than 1/cm² can failto supply a sufficient amount of a nonaqueous electrolyte to the activematerial. A pore density exceeding 10000/cm² can reduce the strength ofthe metal foil in relation to the upper limit of the pore diameter.

The diameter and density of the micropores are measured as follows. Ametal foil is photographed with its back side irradiated with light in adark room, and the photograph is analyzed by image processing to obtainthe diameter and density of the micropores.

The metal foil preferably has a thickness of 1 to 100 μm, stillpreferably 2 to 20 μm, particularly preferably 3 to 10 μm. Metal foilwith a thickness less than 1 μm brings about increased energy densitybut has insufficient mechanical strength and is often difficult toproduce. With a thickness greater than 100 μm, formation of piercingmicropores is not easy, which makes it difficult to increase the energydensity and hinders smooth passage of an electrolyte.

The metal foil can be of various metallic materials. For example, themetal foil contains at least one metal selected from Cu, Ni, Co, Fe, Cr,Sn, Zn, In, Ag, and Au. In other words, the metal foil can be of asingle substance selected from these metals, an alloy of two or moremetals selected from these elements, or a material containing at leastone of these elements and other element(s). A metal foil made of Cu, Ni,Co, Fe, Cr or Au is preferred for its low reactivity with lithium.

A preferred process of preparing a metal foil is described withreference to FIGS. 5(a) through 5(f). First of all, a carrier foil 11 isprepared as shown in FIG. 5(a). The material of the carrier foil 11 isnot particularly limited. The carrier foil 11 is preferablyelectro-conductive. The carrier foil 11 does not need to be made ofmetal as long as it is electro-conductive. Nevertheless, use of ametal-made foil as the carrier foil 11 is advantageous in that thecarrier foil 11, which is left after making metal foil, can be meltedand recycled into foil. In using a metal-made carrier foil 11, it ispreferred to use a carrier foil 11 containing at least one metalselected from Cu, Ni, Co, Fe, Cr, Sn, Zn, In, Ag, Au, Al, and Ti.Considering that the carrier foil 11 is used as a support for makingmetal foil, it is desirable for the carrier foil 11 to have sufficientstrength not to bunch up in the production of the metal foil.Accordingly, the carrier foil 11 preferably has a thickness of about 10to 50 μm.

A coat 12 is formed on one side of the carrier foil 11 by a prescribedmethod as shown in FIG. 5(b). Before formation of the coat, it ispreferred that the surface of the carrier foil 11 be cleaned by apretreatment such as acid cleaning. The coat 12 serves to make thecarrier foil surface, on which metal foil is to be formed, non-uniformin electron conductivity thereby to form a large number of micropores inthe resulting metal foil. The coat 12 is preferably applied to athickness of 0.001 to 1 μm, still preferably 0.002 to 0.5 μm,particularly preferably 0.005 to 0.2 μm. Applied to a thickness in thatrange, the coat 12 covers the surface of the carrier foil 11discontinuously, for example in the form of islands. Discontinuousformation of the coat 12 is advantageous for forming the micropores withthe aforementioned diameter and density more easily. In FIG. 5(b), thesize of the coat 12 is exaggerated for the sake of better understanding.

The coat 12 is made of a material different from the material whichmakes up the metal foil, whereby the resulting metal foil cansuccessfully be peeled from the carrier foil 11 in the step of peelinghereinafter described. It is preferred for the material of the coat 12to differ from the material which makes up metal foil and to contain atleast one element of Cu, Ni, Co, Mn, Fe, Cr, Sn, Zn, In, Ag, Au, C, Al,Si, Ti, and Pd.

The process of forming the coat 12 is not particularly restricted. Forexample, the process of forming the coat 12 can be selected in relationto the process of forming metal foil described infra. More specifically,where the metal foil is to be formed by electroplating, it is preferredto form the coat 12 also by electroplating from the standpoint ofproduction efficiency and the like. The coat 12 can also be formed byother processes, such as electroless plating, sputtering, physical vapordeposition (PVD), chemical vapor deposition (CVD), a sol-gel process,and ion plating.

Where the coat 12 is formed by electroplating, a proper plating bath andplating conditions are decided according to the constituent material ofthe coat 12. For example, in making the coat 12 of tin, a plating bathhaving the composition shown below or a tin borofluoride bath can beused. In using these plating baths, the bath temperature is preferablyabout 15 to 30° C., and the current density is preferably about 0.5 to10 A/dm². SnSO₄ 30 to 70 g/l H₂SO₄ 60 to 150 g/l Cresolsulfonic acid 70to 100 g/l

As stated above, the coat 12 is used to provide the surface of thecarrier foil 11, on which metal foil is to be formed, with non-uniformelectron conductivity. When the constituent material of the coat 12 islargely different from the carrier foil 11 in electron conductivity,application of the coat 12 immediately imparts non-uniformity ofelectron conductivity to the surface on which metal foil is to beformed. Use of carbon as a material of the coat 12 is an example of thatcase. On the other hand, when in using, as a constituent material of thecoat 12, a material whose electron conductivity is about the same asthat of the carrier foil 11, such as various metallic materialsincluding tin, application of the coat 12 does not immediately result innon-uniform electron conductivity of the surface for forming metal foil.Then, in case where the coat 12 is made of such a material, it ispreferred that the carrier foil 11 having the coat 12 formed thereon beexposed to an oxygen-containing atmosphere, such as the air, in a drycondition, thereby to oxidize the surface of the coat 12 (and theexposed area of the carrier foil 11) (see FIG. 5(c)). By this operation,the electron conductivity on the surface for forming metal foil becomesnon-uniform. When electroplating (described infra) is performed on thesurface with the thus created non-uniformity of electron conductivity,there is produced a difference in electrodeposition rate between thesurface of the coat 12 and the exposed area of the carrier foil 11. As aresult, a metal foil having micropores which have the above-reciteddiameter at the above-recited density can easily be formed. The degreeof oxidation is not critical in the present invention. According to thepresent inventors' study, it has been confirmed that allowing thecarrier foil 11 having the coat 12 formed thereon, for example, in theatmosphere for about 10 to 30 minutes is sufficient. However, thecarrier foil 11 having the coat 12 formed thereon may be forciblyoxidized.

The reason why the exposure of the carrier foil 11 having the coat 12formed thereon to an oxygen-containing atmosphere is carried out in adry condition, is for the sake of oxidation efficiency. For example,where the coat 12 is formed by electroplating, it is sufficient to takethe carrier foil 11 out of the plating bath, to dry the carrier foil 11by means of a dryer, etc. and to allow it to stand in the atmosphere fora given time. Where the coat 12 is formed by dry processes, such assputtering and various vacuum deposition techniques, the dryingoperation is unnecessary and the foil 11 having the coat 12 formedthereon is allowed to stand in the atmosphere as it is.

Oxidizing the coat 12 is followed by applying a release agent 13 thereonas shown in FIG. 5(d). The release agent 13 is used to facilitatepeeling the metal foil off the carrier foil in the peeling step whichwill be hereinafter described. An organic compound is preferably appliedas the release agent 13. Nitrogen-containing compounds orsulfur-containing compounds are particularly preferred. Thenitrogen-containing compounds preferably include triazole compounds,such as benzotriazole (BTA), carboxybenzotriazole (CBTA), tolyltriazole(TTA), N′,N′-bis(benzotriazolylmethyl)urea (BTD-U), and3-amino-1H-1,2,4-triazole (ATA). The sulfur-containing compounds includemercaptobenzothiazole (MBT), thiocyanuric acid (TCA), and2-benzimidazolethiol (BIT). Considering that the purpose of applying arelease agent is just to facilitate peeling the formed metal foil offthe carrier foil 11 in the hereinafter described step of peeling, aporous metal foil can be formed without the step of applying a releaseagent.

A material for making metal foil is then deposited on the release layer13 by electroplating to form a metal foil 14 as shown in FIG. 5(e). Theresulting metal foil 14 contains a great number of micropores. WhileFIG. 5(e) shows that the micropores are formed at positions on the topof the individual islands (the coat 12), the aim of this depiction isonly for the sake of convenience. In fact, the micropores are not alwaysformed at positions on the top of the individual islands (the coat 12).The plating bath and plating conditions are chosen appropriatelyaccording to the material of the metal foil. In making a metal foil 14of Ni, for instance, a Watts bath having the composition shown below ora sulfamic acid bath can be used as a plating bath. In using thesebaths, the bath temperature is preferably about 40 to 70° C., and thecurrent density is preferably about 0.5 to 20 A/dm². NiSO₄.6H₂O 150 to300 g/l NiCl₂.6H₂O 30 to 60 g/l H₃BO₃ 30 to 40 g/l

The above-described process of preparing a metal foil 14 is advantageousin that the diameter and the density of micropores can be controlledwith ease for the following reason. In the process, a fresh carrier foilis used for every lot of metal foil. That is, a metal foil is alwayselectrodeposited on a fresh surface so that the condition of the surfaceon which metal foil is formed can be maintained as constant.

Being so thin as previously stated, the resulting metal foil 14 is oftendifficult to handle by itself. This being the case, it is advisable toleave the metal foil 14 remaining on the carrier foil 11 untilprescribed processing operations (such as formation of an activematerial layer as described later) on the metal foil 14 complete. Themetal foil 14 is peeled from the carrier foil 11 as depicted in FIG.5(f) after completion of the prescribed processing operations. Since therelease agent 13 has been applied between the carrier foil 11 and themetal foil 14 as described, peeling of the metal foil 14 from thecarrier foil 11 can be achieved very smoothly. Although FIG. 5(f) showsthat the coat 12 remains on the side of the carrier foil 11 afterpeeling, it depends on the circumstances whether the coat 12 actuallyremains on the carrier foil side or the metal foil side. The sameapplies to the release agent. On whichever side the coat 12 remains, thecoat and the release agent give no adverse influences on the metal foilin view of their very small amounts.

Instead of the above-described process, the metal foil 14 can also beprepared by the following process (hereinafter called an alternativeprocess (1)). In the alternative process (1), a coating, e.g., paste,containing carbonaceous material particles is prepared. Usefulcarbonaceous materials include acetylene black. In order to easily formmicropores with the recited diameter at the recited density, it ispreferred for the carbonaceous material to have an average particle sizeD₅₀ (determined by a laser diffraction scattering method combined withscanning electron microscopic observation) of about 2 to 200 nm,particularly about 10 to 100 nm. The coating is applied to a prescribedsupport. The coating thickness is preferably about 0.001 to 1 μm, stillpreferably about 0.05 to 0.5 μm. A material of the metal foil is thendeposited on the coating layer by electroplating to form metal foil. Theconditions of the electroplating can be the same as those used in theabove-described process.

The support used here typically includes, but is not limited to, theaforementioned carrier foil.

After the formation of metal foil, the metal foil may be eitherseparated from the support or left on the support as formed. Forexample, when the process is applied to the manufacture of a negativeelectrode for a nonaqueous secondary battery, there is no need toseparate the metal foil. In contrast, where the metal foil is to beseparated, it is advisable to apply a release agent on the coating layerformed by applying the carbonaceous material-containing paste and thento electrodeposit the metal foil thereon so as to facilitate peeling.The release agent that can be used includes those usable in theabove-described process.

The metal foil can also be obtained by the following process(hereinafter called an alternative process (2)) instead of thealternative process (1). In the alternative process (2), a plating bathcontaining the material of metal foil is prepared. In making Ni foil,for instance, the aforementioned Watts bath or sulfamic acid bath isprepared. Particles of a carbonaceous material are added and suspendedin the plating bath. The kind and the particle size of the carbonaceousmaterial can be chosen from those useful in the alternative process (1).For easy formation of micropores with the recited diameter at therecited density, the amount of the carbonaceous material to be suspendedin the plating bath is preferably about 0.5 to 50 g/l, still preferablyabout 1 to 10 g/l.

A prescribed support is electroplated in the plating bath while stirringthe bath to keep the carbonaceous material suspended. The materialconstituting metal foil is thus electrodeposited to form a metal foil14. The same support as useful in the alternative process (1) is usableas well. The metal foil thus formed can be handled in the same manner asfor the one obtained by the alternative process (1). If desired, thesupport may have a release agent applied thereto to successfullyseparate the formed metal foil.

The negative electrode using the porous metal foil 14 is produced asfollows. The negative electrode can be produced by taking advantage ofthe above-described processes of preparing metal foil. For example,metal foil is prepared in accordance with the process shown in FIGS.5(a) through 5(e). With the metal foil remaining on the carrier foil, anactive material layer is formed on the metal foil. The active materiallayer is formed by applying, for example, paste containing activematerial particles and electro-conductive material particles. The metalfoil having the active material layer formed thereon is immersed in aplating bath containing an electro-conductive material having lowcapability of forming a lithium compound. In this state, the activematerial layer is electroplated with the conductive material to form asurface coating layer. Finally, the metal foil is separated from thecarrier foil as shown in FIG. 5(f) to obtain a negative electrode.

The thus obtained negative electrode of the invention is assembledtogether with a known positive electrode, separator, and nonaqueouselectrolyte into a nonaqueous secondary battery. A positive electrode isobtained as follows. A positive electrode active material and, ifnecessary, a conductive material and a binder are suspended in anappropriate solvent to prepare a positive electrode active materialmixture, which is applied to a current collector, dried, rolled, andpressed, followed by cutting and punching. The positive electrode activematerial includes conventionally known ones, such as lithium-nickelcomposite oxide, lithium-manganese composite oxide, and lithium-cobaltcomposite oxide. Preferred separators include a nonwoven fabric ofsynthetic resins and a porous film of polyethylene or polypropylene. Thenonaqueous electrolyte used in a lithium secondary battery, for example,is a solution of a lithium salt, which is a supporting electrolyte, inan organic solvent. The lithium salt includes LiClO₄, LiAlCl₄, LiPF₆,LiAsF₆, LiSbF₆, LiSCN, LiCl, LiBr, LiI, LiCF₃SO₃, and LiC₄F₉SO₃.

The present invention is not limited to the aforementioned embodiments.For example, punching metal or expanded metal having a great number ofopenings or metal foam, such as nickel foam, can be used as a currentcollector. In using punching metal or expanded metal, the opening areais preferably 0.0001 to 4 mm², still preferably 0.002 to 1 mm². Wherepunched metal or expanded metal is used, the active material layer isformed preferentially in the openings, and the surface coating layer isformed on the surface of the thus formed active material layer and thesurface of the punching metal or expanded metal. On the other hand,where metal foam is used, the cells of the foamed body are filled withthe active material layer, and the surface coating layer is formed onthe surface of the active material layer and the surface of the metalfoam.

While the cross-sectional photographs shown in FIGS. 2 to 4 represent anembodiment in which the active material structure 5 is formed on onlyone side of the current collector 2, the active material structure maybe formed on both sides of the current collector.

The present invention will now be illustrated in greater detail withreference to Examples, but it should be understood that the invention isnot construed as being limited thereto. Unless otherwise noted, all thepercents are by weight.

EXAMPLE 1-1

(1) Preparation of Active Material Particles

A molten metal at 1400° C. containing 90% of silicon and 10% of nickelwas cast into a copper-made mold and quenched to obtain an ingot of asilicon-nickel alloy. The ingot was pulverized and sieved to obtainsilicon-nickel alloy particles having particle sizes of 0.1 to 10 μm.The silicon-nickel alloy particles and nickel particles (particle size:30 μm) were blended at a rate of 80% 20% and mixed and pulverizedsimultaneously in an attritor to obtain uniformly mixed particles ofsilicon-nickel particles and nickel. The mixed particles had the maximumparticle size of 1 μm and a D₅₀ value of 0.8 μm.

(2) Preparation of Slurry

A slurry having the following composition was prepared. Mixed particlesobtained in (1) above 16% Acetylene black (particle size: 0.1 μm) 2%Binder (polyvinylidene fluoride) 2% Diluting solvent(N-methylpyrrolidone) 80%(3) Formation of Active Material Layer

The above prepared slurry was applied to a 35 μm thick copper foil anddried to form an active material layer having a dry thickness of 60 μm.The active material layer was densified by prepressing.

(4) Formation of Surface Coating Layer

The copper foil having the active material layer formed thereon wasimmersed in a plating bath having the following composition to carry outelectroplating. Nickel 50 g/l Sulfuric acid 60 g/l Bath temperature 40°C.

After formation of the surface coating layer, the copper foil was takenout of the plating bath, and both the active material layer and thesurface coating layer were densified by roll pressing. The thickness ofthe thus formed active material structure was found to be 23 μm as aresult of electron microscopic observation. As a result of chemicalanalysis, the amounts of the active material particles and acetyleneblack were found to be 40% and 5%, respectively. Presence of microporesin the resulting negative electrode was confirmed by observation underan electron microscope.

EXAMPLES 1-2 TO 1-4

A negative electrode was produced in the same manner as in Example 1-1,except for using the active material particles shown in Table 1-1 below.The same electron microscopic observation as in Example 1-1 revealedpresence of micropores in the resulting negative electrode.

EXAMPLE 1-5

A 35 μm thick copper foil was plated with nickel to a deposit thicknessof 2 μm to prepare a current collector. An active material layer and asurface coating layer were formed on the nickel layer in the same manneras in Example 1-1, except for using the active material particles shownin Table 1-1 in the active material layer. The same electron microscopicobservation as in Example 1-1 revealed presence of micropores in theresulting negative electrode.

EXAMPLES 1-6

A 400 μm thick nickel foam was used as a current collector. The nickelfoam had an average cell diameter of 20 μm. A slurry was prepared in thesame manner as in Example 1-1, except for using the active materialparticles shown in Table 1-1. The nickel foam was impregnated with theslurry. The impregnated foam was immersed in the same plating bath asused in Example 1-1 to carry out electroplating. The same electronmicroscopic observation revealed presence of micropores in the resultingnegative electrode.

EXAMPLE 1-7

A 40 μm thick expanded copper metal sheet was used as a currentcollector. The area of the individual openings of the expanded metal was0.01 mm². A slurry was prepared in the same manner as in Example 1-1,except for using the active material particles shown in Table 1-1, andthe expanded metal was impregnated with the slurry. The impregnatedexpanded metal was immersed in the same plating bath as used in Example1-1 to carry out electroplating. The same electron microscopicobservation revealed presence of micropores in the resulting negativeelectrode.

M-306[0051]

COMPARATIVE EXAMPLE 1-1

Graphite powder having a particle size of 10 μm, a binder (PVDF), and adiluting solvent (N-methylpyrrolidone) were kneaded to prepare a slurry.The slurry was applied to a 30 μm thick copper foil, dried, and pressedto obtain a negative electrode. The pressed graphite layer was 20 μmthick.

COMPARATIVE EXAMPLE 1-2

A negative electrode was obtained in the same manner as in ComparativeExample 1-1, except for replacing graphite powder with silicon powderhaving a particle size of 5 μm.

Evaluation of Performance:

A nonaqueous secondary battery was assembled using each of the negativeelectrodes prepared in Examples and Comparative Examples as follows. Thebattery was evaluated in irreversible capacity, capacity density perunit volume when charged, charge/discharge efficiency in the 10th cycle,and capacity retention in the 50th cycle in accordance with thefollowing methods. The results of evaluation are shown in Table 1-1.

1) Preparation of Nonaqueous Secondary Battery

A metallic lithium as a counter electrode and the negative electrodeobtained above as a working electrode were placed to face each otherwith a separator between them and assembled into a nonaqueous secondarybattery in a usual manner by using an LiPF₆ solution in a mixture ofethylene carbonate and diethyl carbonate (1:1 by volume) as a nonaqueouselectrolyte.

2) Irreversible Capacity

An irreversible capacity, represented by equation shown below, indicatesthe part of the charge capacity that is not discharged and remains inthe active material.Irreversible capacity (%)=(1−first discharge capacity/first chargecapacity)×1003) Capacity Density

The first discharge capacity (mAh/g).

4) Charge/discharge efficiency in the 10th cycleCharge/discharge efficiency in 10th cycle (%) =discharge capacity in10th cycle/charge capacity in 10th cycle×1005) Capacity retention in the 50th cycleCapacity retention (50th cycle) (%)=discharge capacity (50thcycle)/maximum discharge capacity×100

TABLE 1-1 Active Material Structure Active Material Layer Charge/Surface Coating Active Material Particles Discharge Layer Size Contentin Irreversible Capacity Efficiency Capacity Thickness Thickness D₅₀Structure Capacity Density at 10th Retention in (μm) Material (μm) (μm)(wt %) Material (%) (mAh/g) cycle (%) 50th Cycle (%) Example 1-1 3 Ni 200.8 40 [Si90/Ni10 4 3100 99.9 98 (cast)]80/Ni20 Example 1-2 3 Ni 20 0.840 [Si90/Ni10 5 3100 99.9 98 (cast)]80/Cu20 Example 1-3 3 Ni 20 0.8 40[Si80/Cu20 4 2800 99.9 97 (cast)]80/Ni20 Example 1-4 3 Ni 20 0.8 40[Si80/Cu20 5 2800 99.9 96 (cast)]80/Cu20 Example 1-5 3 Ni 20 0.8 40[Si80/Ni20 4 2800 99.9 99 (cast)]80/Ni20 Example 1-6 3 Ni 20 0.8 40[Si80/Ni20 4 2800 99.9 99 (cast)]80/Ni20 Example 1-7 3 Ni 20 0.8 40[Si80/Ni20 4 2800 99.9 99 (cast)]80/Ni20 Compara. — 10 80 graphite 10310 99.7 100 Example 1-1 Compara. — 5 80 pure Si 60 2000 85.0 7 Example1-2

As is apparent from the results shown in Table 1-1, the secondarybatteries using the negative electrodes of Examples each have a lowerirreversible capacity and a higher capacity density and charge/dischargeefficiency than those using the negative electrodes of ComparativeExamples. They also exhibit high capacity retention. While not shown inthe Table, the negative electrodes of Examples 1-1 to 1-7 had astructure as shown in FIG. 2 under observation with an electronmicroscope.

EXAMPLE 2-1

(1) Preparation of Slurry

A slurry having the following composition was prepared. Tin particles(particle size D₅₀: 2 μm) 16% Acetylene black (particle size: 0.1 μm) 2%Binder (polyvinylidene fluoride) 2% Diluting solvent(N-methylpyrrolidone) 80%(2) Formation of Coating

The above prepared slurry was applied to a 30 μm thick copper foil anddried. The thickness of the dried coating was 60 μm.

(3) Formation of Coating Layer

The copper foil having the coating formed thereon was immersed in aplating bath having the following composition to carry outelectroplating. Copper 50 g/l Sulfuric acid 60 g/l Bath temperature 40°C.

After formation of the coating layer, the copper foil was taken out ofthe plating bath, and the coating layer, including the coating, wasdensified by roll pressing. The thickness of the resulting coating layerwas found to be 20 μm as a result of electron microscopic observation.As a result of chemical analysis, the amounts of tin particles andacetylene black in the coating layer were found to be 70% and 5%,respectively.

EXAMPLES 2-2 AND 2-3

A negative electrode was obtained in the same manner as in Example 2-1,except that the coating layer was formed of nickel (Example 2-2) orcobalt (Example 2-3).

EXAMPLE 2-4

A molten metal containing 60% tin and 40% copper at 1000° C. wasinjected onto the peripheral surface of a copper roll rotating at a highspeed (1000 rpm). The injected molten roll is quenched on the roll intoa thin tin-copper alloy strip. The cooling rate was 10³ K/sec or higher.The strip was ground and sieved to obtain particles having particlesizes of 0.1 to 10 μm. A negative electrode was obtained in the samemanner as in Example 2-1, except for using the resulting alloyparticles.

EXAMPLES 2-5 AND 2-6

A negative electrode was obtained in the same manner as in Example 2-4,except for using tin-copper alloy particles having the composition shownin Table 2-1 below.

EXAMPLES 2-7 AND 2-8

A negative electrode was obtained in the same manner as in Example 2-4,except for using tin-nickel alloy particles having the composition shownin Table 2-1.

EXAMPLES 2-9 AND 2-10

A negative electrode was obtained in the same manner as in Example 2-4,except for using tin-copper-nickel alloy particles having thecomposition shown in Table 2-1.

EXAMPLES 2-11 TO 2-16

A negative electrode was obtained in the same manner as in Example 2-4,except for using tin-based ternary alloy particles having thecomposition shown in Table 2-1 prepared by a quenching process.

EXAMPLE 2-17

Tin particles (particle size: 30 μm) 90% and copper particles (particlesize: 30 μm) 10% were mixed and pulverized simultaneously in an attritorto obtain uniformly mixed tin/copper powder having particle sizes of 0.1 to 10 μm (D₅₀: 2 μm). A negative electrode was obtained in the samemanner as in Example 2-1, except for using the resulting mixed powder.

EXAMPLES 2-18 TO 2-31

A negative electrode was obtained in the same manner as in Example 2-17,except for using tin-copper mixed powder of the composition and particlesize shown in Table 2-2 below or changing the thickness of the coatinglayer or the content of the mixed powder in the coating layer as shownin Table 2-2.

EXAMPLES 2-32 TO 2-39

A negative electrode was obtained in the same manner as in Example 2-17,except for using tin-based mixed powder of the composition shown inTable 2-2.

EXAMPLE 2-40

A molten metal at 1000° C. containing 75% tin and 25% copper wasinjected onto the peripheral surface of a copper roll rotating at a highspeed (1000 rpm). The injected molten roll is quenched on the roll intoa thin tin-copper alloy strip. The cooling rate was 10³ K/sec or higher.The strip was ground and sieved to obtain particles -having particlesizes of 0.1 to 10 μm. The resulting alloy particles 99% and silverparticles (particle size: 30 μm) 1% were mixed and pulverizedsimultaneously in an attritor to obtain uniformly mixed tin-copperalloy/silver powder having particle sizes of 0.1 to 10 μm (D₅₀: 2 μm). Anegative electrode was obtained in the same manner as in Example 2-1,except for using the resulting mixed powder.

EXAMPLES 2-41 TO 2-48

A negative electrode was obtained in the same manner as in Example 2-40,except for using mixed powder obtained by mixing the tin-copper alloyparticles shown in Table 2-3 below and silver or copper particles in themixing ratio shown in the Table.

EXAMPLE 2-49

Tin particles having particle sizes of 0.1 to 10 μm were electrolessplated in a plating bath having the tin particles suspended therein andcontaining copper sulfate and Rochelle salt to obtain copper-coated tinparticles. The concentrations of the tin particles, copper sulfate, andRochelle salt in the plating bath were 500 g/l, 7.5 g/l, and 85 g/l,respectively. The plating bath had a pH of 12.5 and a temperature of 25°C. Formaldehyde was used as a reducing agent and its concentration is 22cc/l. A negative electrode was obtained in the otherwise same manner asin Example 2-1.

EXAMPLES 2-50 TO 2-53

A negative electrode was obtained in the same manner as in Example 2-41,except for using copper-coated tin particles (Examples 2-50 and 2-51) ornickel-coated tin particles (Examples 2-52 and 2-53) each having thecomposition shown in Table 2-3 and obtained by electroless plating.

COMPARATIVE EXAMPLE 2-1

A negative electrode was obtained in the same manner as in ComparativeExample 1-1, except for replacing the graphite powder with tin particleshaving a particle size of 5 μm.

Performance Evaluation:

Nonaqueous secondary batteries were assembled using each of the negativeelectrodes prepared in Examples and Comparative Examples in the samemanner as described supra. The battery was evaluated in irreversiblecapacity, capacity density per unit volume when charged,charge/discharge efficiency in the 10th cycle, and capacity retention inthe 50th cycle in accordance with the methods described supra. Theresults of evaluation are shown in Tables 2-1 to 2-3. TABLE 2-1 Negativeelectrode Active Material Charge/ Capacity Coating Layer Content inIrreversible Capacity Discharge Retention Example Thickness PlatingParticle Size Coating Layer Material (Kind of Active Capacity DensityEfficiency at 10th at 50th No. (μm) Material D₅₀ (μm) (wt %) Material)*¹(%) (mAh/g) Cycle (%) Cycle (%) 2-1 20 Cu 2 70 pure Sn 9 950 99.6 99 2-220 Ni 2 70 pure Sn 14 910 99.3 96 2-3 20 Co 2 70 pure Sn 13 900 99.1 962-4 20 Cu 2 70 Sn60/Cu40(alloy) 6 450 99.9 98 2-5 20 Cu 2 70Sn75/Cu25(alloy) 6 650 99.9 94 2-6 20 Cu 2 70 Sn90/Cu10(alloy) 6 85099.9 99 2-7 20 Cu 2 70 Sn80/Ni20(alloy) 6 500 99.9 92 2-8 20 Cu 2 70Sn95/Ni15(alloy) 6 850 99.9 99 2-9 20 Cu 2 70 Sn80/Cu10/Ni10 6 750 99.997 (alloy) 2-10 20 Cu 2 70 Sn85/Cu10/Ni5 6 800 99.9 97 (alloy) 2-11 20Cu 2 70 Sn80/Cu19.5/Al0.5 (alloy) 8 870 99.9 92 2-12 20 Cu 2 70Sn80/Cu19.5/Ni0.5 9 810 99.9 96 (alloy) 2-13 20 Cu 2 70Sn80/Cu19.5/Co0.5 9 800 99.9 98 (alloy) 2-14 20 Cu 2 70Sn80/Cu19.5/Ti0.5 8 820 99.9 94 (alloy) 2-15 20 Cu 2 70Sn80/Cu19.5/La0.5 9 860 99.9 97 (alloy) 2-16 20 Cu 2 70Sn80/Cu19.5/Ce0.5 9 860 99.9 98 (alloy)*¹Figures indicate % by weight.

TABLE 2-2 Negative electrode Active Material Charge/ Coating LayerContent in Irreversible Capacity Discharge Capacity Example ThicknessPlating Particle Size Coating Layer Material (Kind of Active CapacityDensity Efficiency at Retention at No. (μm) Material D₅₀ (μm) (wt %)Material)*¹ (%) (mAh/g) 10th Cycle (%) 50th Cycle (%) 2-17 20 Cu 2 70Sn90 + Cu10 7 880 99.9 97 (mixed powder) 2-18 20 Cu 0.5 70 Sn90 + Cu10 8880 99.9 97 (mixed powder) 2-19 20 Cu 10 70 Sn90 + Cu10 7 880 99.9 92(mixed powder) 2-20 20 Cu 0.2 70 Sn90 + Cu10 7 880 99.10 93 (mixedpowder) 2-21 20 Cu 1 70 Sn90 + Cu10 7 880 99.11 97 (mixed powder) 2-2220 Cu 5 70 Sn90 + Cu10 7 880 99.12 95 (mixed powder) 2-23 20 Cu 20 70Sn90 + Cu10 7 880 99.13 98 (mixed powder) 2-24 20 Cu 2 30 Sn90 + Cu10 7880 99.9 94 (mixed powder) 2-25 20 Cu 2 50 Sn90 + Cu10 7 880 99.9 99(mixed powder) 2-26 5 Cu 1 70 Sn90 + Cu10 8 880 99.9 99 (mixed powder)2-27 10 Cu 2 70 Sn90 + Cu10 7 880 99.9 96 (mixed powder) 2-28 15 Cu 2 70Sn90 + Cu10 7 880 99.9 97 (mixed powder) 2-29 20 Cu 2 70 Sn60 + Cu40 8590 99.9 98 (mixed powder) 2-30 20 Cu 2 70 Sn75 + Cu25 8 740 99.9 98(mixed powder) 2-31 20 Cu 2 70 Sn95 + Cu5 8 890 99.9 92 (mixed powder)2-32 20 Cu 2 70 Sn99 + Ag1 5 900 99.9 93 (mixed powder) 2-33 20 Cu 2 70Sn95 + Ag5 5 890 99.9 96 (mixed powder) 2-34 20 Cu 2 70 Sn90 + Ag10 5870 99.9 94 (mixed powder) 2-35 20 Cu 2 70 Sn80 + Ag20 5 850 99.9 92(mixed powder) 2-36 20 Cu 2 60 Sn90 + Si10 7 980 99.9 92 (mixed powder)2-37 20 Cu 2 60 Sn50 + Si50 7 2500 99.9 93 (mixed powder) 2-38 20 Cu 260 Sn50 + Si40 + Cu10 7 2200 99.9 96 (mixed powder) 2-39 20 Cu 2 60Sn50 + Si40 + Cu10 7 2200 99.9 97 (mixed powder)*¹Figures indicate % by weight.

TABLE 2-3 Negative electrode Active Material Charge/ Capacity CoatingLayer Content in Irreversible Capacity Discharge Retention at ExampleThickness Plating Particle Size Coating Layer Material (Kind of ActiveCapacity Density Efficiency at 50th No. (μm) Material D₅₀ (μm) (wt %)Material)*¹ (%) (mAh/g) 10th Cycle (%) Cycle (%) 2-40 20 Cu 2 70[Sn75/Cu25]99 + Ag1 5 690 99.9 94 (mixed powder) 2-41 20 Cu 2 70[Sn75/Cu25]95 + Ag5 5 680 99.9 98 (mixed powder) 2-42 20 Cu 2 70[Sn75/Cu25]90 + Ag10 5 670 99.9 99 (mixed powder) 2-43 20 Cu 2 70[Sn75/Cu25]80 + Ag20 5 640 99.9 99 (mixed powder) 2-44 20 Cu 2 70[Sn75/Cu25]99 + Cu1 5 680 99.9 93 (mixed powder) 2-45 20 Cu 2 70[Sn75/Cu25]95 + Cu5 5 670 99.9 96 (mixed powder) 2-46 20 Cu 2 70[Sn75/Cu25]90 + Cu10 5 620 99.9 97 (mixed powder) 2-47 20 Cu 2 70[Sn75/Cu25]80 + Cu20 5 600 99.9 93 (mixed powder) 2-48 20 Cu 2 70[Sn75/Cu25]60 + Cu40 5 450 99.9 94 (mixed powder) 2-49 20 Cu 2 70Sn80/Cu20 5 790 99.9 99 (electroless plating) 2-50 20 Cu 2 70 Sn95/Cu5(electroless 5 910 99.9 98 Plating) 2-51 20 Cu 2 70 Sn99/Cu1(electroless 6 930 99.9 96 Plating) 2-52 20 Cu 2 70 Sn99/Ni1(electroless 11 900 99.9 96 plating) 2-53 20 Cu 2 70 Sn99.5/Ni0.05(electroless 7 930 99.9 95 plating) Comp. no plating 5 80 pure Sn 20 95095.0 7 Ex. 2-1*¹Figures indicate % by weight.

As is apparent from the results shown in Tables 2-1 to 2-3, thesecondary batteries using the negative electrodes obtained in Examplesretain the same levels of irreversible capacity, charge/dischargeefficiency and capacity retention as the comparative secondary batteryusing the comparative negative electrode and also have extremely highercapacity density than the comparative battery.

EXAMPLE 3-1

Silicon particles (particle size D₅₀: 5 μm) 600 g/l Copper sulfate  50g/l Sulfuric acid  70 g/l Cresolsulfonic acid  70 g/l Gelatin  2 g/lβ-Naphthol  1.5 g/l (2) Dispersion Plating

A 30 μm thick copper foil was immersed in the plating bath, in which thesilicon particles were suspended, at 20° C. and electroplated at acurrent density of 10 A/dm². There was thus formed an active materiallayer having silicon particles uniformly dispersed therein and a surfacecoating layer covering the active material layer. As a result ofelectron microscopic observation, the active material structurecontaining the active material layer and the surface coating layer wasfound to be 35 μm. Chemical analysis revealed that the silicon powdercontent in the active material structure was 30%.

EXAMPLE 3-2

(1) Preparation of Slurry

A slurry having the following composition was prepared. Siliconparticles (D₅₀: 5 μm) 16% Acetylene black (particle size: 0.1 μm) 2%Binder (polyvinylidene fluoride) 2% Diluting solvent(N-methylpyrrolidone) 80%(2) Formation of Active Material Layer

The above prepared slurry was applied to a 30 μm thick copper foil anddried to form an active material layer having a dry thickness of 60 μm.

(3) Formation of Surface Coating Layer

The copper foil having the active material layer formed thereon wasimmersed in a plating bath having the following composition to carry outelectroplating. Copper 50 g/l Sulfuric acid 60 g/l Bath temperature 40°C.

After forming the surface coating layer, the copper foil was taken outof the plating bath, and both the active material layer and the surfacecoating layer were densified by roll pressing. The thickness of the thusformed active material structure was found to be 30 μm as a result ofelectron microscopic observation. As a result of chemical analysis, theamounts of the silicon particles and acetylene black in the activematerial structure were found to be 35% and 5%, respectively.

EXAMPLES 3-3 AND 3-4

A negative electrode was obtained in the same manner as in Example 3-2,except for forming the coating layer of nickel (Example 3-3) or cobalt(Example 3-4).

EXAMPLE 3-5

A molten metal at 1400° C. containing 50% of silicon and 50% of copperwas cast into a copper-made mold and quenched to obtain an ingot of asilicon-copper alloy. The ingot was pulverized and sieved to obtainalloy particles having particle sizes of 0.1 to 10 μm. A negativeelectrode was obtained in the same manner as in Example 3-2, except forusing the resulting alloy particles.

EXAMPLES 3-6 TO 3-8

A negative electrode was obtained in the same manner as in Example 3-5,except for using silicon-copper alloy particles of the composition shownin Table 3-1 below.

EXAMPLES 3-9 TO 3-11

A negative electrode was obtained in the same manner as in Example 3-5,except for using silicon-nickel alloy particles of the composition shownin Table 3-1.

EXAMPLES 3-12 AND 3-13

A negative electrode was obtained in the same manner as in Example 3-5,except for using silicon-copper-nickel alloy particles of thecomposition shown in Table 3-1.

EXAMPLES 3-14

Silicon particles (particle size: 100 μm) 80% and copper particles(particle size: 30 μm) 20% were mixed and pulverized simultaneously inan attritor to obtain uniformly mixed silicon/copper powder havingparticle sizes of 2 to 10 μm (D₅₀: 5 μm). A negative electrode wasobtained in the same manner as -in Example 3-2, except for using theresulting mixed powder.

EXAMPLES 3-15 TO 3-26

A negative electrode was obtained in the same manner as in Example 3-14,except for using silicon-copper mixed powder of the composition andparticle size shown in Table 3-2 below and changing the thickness of theactive material structure as shown in the Table.

EXAMPLE 3-27

Silicon particles having particle sizes of 0.2 to 8 μm were electrolessplated with copper in a plating bath containing copper sulfate andRochelle salt, in which the silicon particles were suspended, to obtaincopper-coated silicon particles. The concentrations of the siliconparticles, copper sulfate, and Rochelle salt in the plating bath were500 g/l, 7.5 g/l, and 85 g/l, respectively. The plating bath had a pH of12.5 and a temperature of 25° C. As a reducing agent formaldehyde wasused in a concentration of 22 cc/l. A negative electrode was obtained inthe otherwise same manner as in Example 3-2.

EXAMPLES 3-28 TO 3-31

A negative electrode was obtained in the same manner as in Example 3-18,except for using copper-coated silicon particles (Examples 3-28 and3-29) or nickel-coated silicon particles (Examples 3-30 and 3-31) eachhaving the composition shown in Table 3-2 and obtained by electrolessplating.

EXAMPLES 3-32 TO 3-37

A negative electrode was obtained in the same manner as in Example 3-5,except for using silicon-based ternary alloy particles having thecomposition shown in Table 3-3 below and prepared by a quenchingprocess.

EXAMPLE 3-38

Silicon particles (particle size: 100 μm) 20% and graphite particles(D₅₀: 20 μm) 80% were mixed and pulverized simultaneously by mechanicalmilling to obtain uniformly mixed silicon/graphite powder having aparticle size (D₅₀) of 0.5 μm. A negative electrode was obtained in thesame manner as in Example 3-2, except for using the resulting mixedpowder and forming the surface coating layer of nickel.

EXAMPLES 3-39 TO 3-42

A negative electrode was obtained in the same manner as in Example 3-38,except for using mixed powder of the composition shown in Table 3-3.

EXAMPLE 3-43

A negative electrode was obtained in the same manner as in Example 3-5,except for using alloy powder composed of silicon 80%, copper 19%, andlithium 1% and forming the surface coating layer of nickel.

Evaluation of Performance:

A nonaqueous secondary battery was assembled using each of the negativeelectrodes prepared in Examples and Comparative Examples in the samemanner as described supra. The battery was evaluated in irreversiblecapacity, capacity density per unit volume when charged,charge/discharge efficiency in the 10th cycle, and capacity retention inthe 50th cycle in accordance with the methods described supra. Theresults of evaluation are shown in Tables 3-1 through 3-3. TABLE 3-1Active Material Structure Charge/ Surface Coating Si-based ActiveMaterial Layer Discharge Capacity Layer Size Content in IrreversibleCapacity Efficiency at Retention in Example Thickness ThicknessThickness D₅₀ Structure Capacity Density 10th Cycle 50th Cycle No. (μm)(μm) Material (μm) (μm) (wt %) Material (%) (mAh/g) (%) (%) 3-1 30 5 Cu25 5 30 pure Si 12 4010 99.6 95 3-2 30 5 Cu 25 5 35 pure Si 9 4010 99.795 3-3 30 5 Ni 25 5 35 pure Si 14 3700 99.5 96 3-4 30 5 Co 25 5 35 pureSi 13 3600 99.5 96 3-5 30 5 Cu 25 5 45 Si50/Cu50 5 2000 99.7 99 3-6 30 5Cu 25 5 40 Si60/Cu40 6 2400 99.7 99 3-7 30 5 Cu 25 5 40 Si70/Cu30 6 280099.7 99 3-8 30 5 Cu 25 5 38 Si80/Cu20 6 3200 99.7 99 3-9 30 5 Cu 25 5 45Si50/Ni50 6 500 99.7 99 3-10 30 5 Cu 25 5 40 Si65/Ni35 6 800 99.7 993-11 30 5 Cu 25 5 35 Si80/Ni20 6 1300 99.7 99 3-12 30 5 Cu 25 5 40Si60/Cu20/Ni20 6 2000 99.7 99 3-13 30 5 Cu 25 5 40 Si70/Cu15/Ni15 6 230099.7 99

TABLE 3-2 Active Material Structure Charge/ Surface Coating Si-basedActive Material Layer Discharge Capacity Layer Content in IrreversibleCapacity Efficiency at Retention Example Thickness Thickness ThicknessSize D₅₀ Structure Capacity Density 10th Cycle in 50th No. (μm) (μm)Material (μm) (μm) (wt %) Material (%) (mAh/g) (%) Cycle (%) 3-14 30 5Cu 25 5 38 Si80/Cu20 7 3200 99.7 99 3-15 30 5 Cu 25 0.8 38 Si80/Cu20 83200 99.9 100 3-16 30 5 Cu 25 10 38 Si80/Cu20 7 3200 99.7 96 3-17 30 5Cu 25 20 38 Si80/Cu20 7 3200 99.6 95 3-18 30 5 Cu 25 5 10 Si80/Cu20 73200 99.8 99 3-19 30 5 Cu 25 5 20 Si80/Cu20 7 3200 99.7 99 3-20 5 1 Cu 41 38 Si80/Cu20 8 3200 99.8 99 3-21 10 2 Cu 8 5 38 Si80/Cu20 7 3200 99.799 3-22 15 3 Cu 12 5 38 Si80/Cu20 7 3200 99.7 99 3-23 20 4 Cu 16 5 38Si80/Cu20 7 3200 99.7 99 3-24 30 5 Cu 25 5 37 Si90/Cu10 8 3600 99.7 983-25 30 5 Cu 25 5 36 Si95/Cu5 8 3600 99.7 98 3-26 30 5 Cu 25 5 36Si95/Ni5 9 3500 99.7 98 3-27 30 5 Cu 25 5 35 Si80/Cu20 5 3200 99.7 993-28 30 5 Cu 25 5 35 Si80/Cu20 5 3800 99.7 99 3-29 30 5 Cu 25 5 35Si99/Cu1 6 3900 99.7 99 3-30 30 5 Cu 25 5 35 Si99/Ni1 11 3900 99.7 993-31 30 5 Cu 25 5 35 Si99.5/Ni0.05 7 4000 99.7 99

TABLE 3-3 Active Material Structure Charge/ Surface Coating Si-basedActive Material Layer Discharge Capacity Layer Size Content inIrreversible Capacity Efficiency at Retention Example ThicknessThickness Thickness D₅₀ Structure Capacity Density 10th Cycle in 50thNo. (μm) (μm) Material (μm) (μm) (wt %) Material (%) (mAh/g) (%) Cycle(%) 3-32 30 5 Cu 25 5 38 Si80/Cu19.5/Al0.5 8 3100 99.7 99 3-33 30 5 Cu25 5 38 Si80/Cu19.5/Ni0.5 9 3100 99.7 99 3-34 30 5 Cu 25 5 38Si80/Cu19.5/Co0.5 9 3000 99.7 99 3-35 30 5 Cu 25 5 38 Si80/Cu19.5/Ti0.58 3100 99.7 99 3-36 30 5 Cu 25 5 38 Si80/Cu19.5/La0.5 9 3060 99.7 993-37 30 5 Cu 25 5 38 Si80/Cu19.5/Ce0.5 9 3070 99.7 99 3-38 30 5 Ni 250.5 35 Si20/C80 6 1040 99.8 100 3-39 30 5 Ni 25 0.5 35 Si40/C60 6 174099.8 97 3-40 30 5 Ni 25 0.5 35 Si40/C40/Cu20 6 1600 99.8 97 3-41 30 5 Ni25 0.5 35 Si60/C40 7 2510 99.7 94 3-42 30 5 Ni 25 0.5 35 Si80/C20 8 323099.7 92 3-43 30 5 Ni 25 5 38 Si80/Cu19/Li1 0 3200 100 100

As is apparent from the results shown in Tables 3-1 to 3-3, thesecondary batteries using the negative electrodes obtained in Examplesretain the same levels of irreversible capacity, charge/dischargeefficiency and capacity retention as the comparative secondary batteriesusing the comparative negative electrodes and also have extremely highercapacity density than the comparative batteries.

INDUSTRIAL APPLICABILITY

The negative electrode for nonaqueous secondary batteries according tothe present invention provides a secondary battery having higher energydensity than conventional negative electrodes. The negative electrodefor nonaqueous secondary batteries according to the present inventionprevents the active material from falling off the current collector sothat the current collecting performance of the active material ismaintained against repetition of charge/discharge cycles. Furthermore,the secondary battery using the negative electrode is less susceptibleto deterioration against repetition of charge/discharge cycles andtherefore enjoys a greatly extended service life and an increasedcharge/discharge efficiency.

1. A negative electrode for a nonaqueous secondary battery comprising acurrent collector and an active material structure containing anelectro-conductive material having low capability of forming a lithiumcompound on at least one side of the current collector, the activematerial structure containing 5 to 80% by weight of active materialparticles containing a material having high capability of forming alithium compound.
 2. The negative electrode for a nonaqueous secondarybattery according to claim 1, wherein the active material structure hasan active material layer containing the active material particles and asurface coating layer located on the active material layer.
 3. Thenegative electrode for a nonaqueous secondary battery according to claim1, wherein the material having high capability of forming a lithiumcompound is tin or silicon.
 4. The negative electrode for a nonaqueoussecondary battery according to claim 2, wherein the active materiallayer contains 0.1 to 20% by weight of an electro-conductive carbonmaterial.
 5. The negative electrode for a nonaqueous secondary batteryaccording to claim 2, wherein the material constituting the surfacecoating layer enters the active material layer or reaches the currentcollector.
 6. The negative electrode for a nonaqueous secondary batteryaccording to claim 2, wherein the material constituting the surfacecoating layer penetrates throughout the active material layer.
 7. Thenegative electrode for a nonaqueous secondary battery according to claim2, wherein the surface coating layer has a large number of microporesextending in the thickness direction of the surface coating layer andallowing a nonaqueous electrolyte to pass therethrough.
 8. The negativeelectrode for a nonaqueous secondary battery according to claim 3,wherein the active material particles are particles of single silicon orsingle tin.
 9. The negative electrode for a nonaqueous secondary batteryaccording to claim 3, wherein the active material particles are mixedparticles comprising at least silicon or tin and carbon, the mixedparticles containing 10 to 90% by weight of silicon or tin and 10 to 90%by weight of carbon.
 10. The negative electrode for a nonaqueoussecondary battery according to claim 3, wherein the active materialparticles are mixed particles comprising silicon or tin and a metal, themixed particles containing 30% to 99.9% by weight of silicon or tin and0.1 to 70% by weight of at least one element selected from the groupconsisting of Cu, Ag, Li, Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn (except fora case where the active material particles contain tin), Si (except fora case where the active material particles contain silicon), In, V, Ti,Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd.
 11. The negative electrode fora nonaqueous secondary battery according to claim 3, wherein the activematerial particles are silicon compound particles or tin compoundparticles, the silicon compound particles or the tin compound particlescontaining 30% to 99.9% by weight of silicon or tin and 0.1 to 70% byweight of at least one element selected from the group consisting of Cu,Ag, Li, Ni, Co, Fe, Cr, Zn, B, Al, Ge, Sn (except for a case where theactive material particles contain tin), Si (except for a case where theactive material particles contain silicon), In, V, Ti, Y, Zr, Nb, Ta, W,La, Ce, Pr, Pd, and Nd.
 12. The negative electrode for a nonaqueoussecondary battery according to claim 3, wherein the active materialparticles are mixed particles comprising silicon compound particles ortin compound particles and metal particles, the mixed particlescontaining 30% to 99.9% by weight of silicon compound particles or tincompound particles and 0.1 to 70% by weight of particles of at least oneelement selected from the group consisting of Cu, Ag, Li, Ni, Co, Fe,Cr, Zn, B, Al, Ge, Sn (except for a case where the active materialparticles contain tin), Si (except for a case where the active materialparticles contain silicon), In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd,and Nd, and the compound particles containing 30% to 99.9% by weight ofsilicon or tin and 0.1 to 70% by weight of at least one element selectedfrom the group consisting of Cu, Ag, Li, Ni, Co, Fe, Cr, Zn, B, Al, Ge,Sn (except for a case where the active material particles contain tin),Si (except for a case where the active material particles containsilicon), In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd.
 13. Thenegative electrode for a nonaqueous secondary battery according to claim3, wherein the active material particles are single silicon or singletin particles coated with a metal, the metal being at least one elementselected from the group consisting of Cu, Ag, Ni, Co, Fe, Cr, Zn, B, Al,Ge, Sn (except for a case where the active material particles containtin), Si (except for a case where the active material particles containsilicon), In, V, Ti, Y, Zr, Nb, Ta, W, La, Ce, Pr, Pd, and Nd, and theactive material particles containing 70% to 99.9% by weight of siliconor tin and 0.1 to 30% by weight of the metal.
 14. The negative electrodefor a nonaqueous secondary battery according to claim 1, wherein theactive material particles have a maximum particle size of 50 μm orsmaller.
 15. The negative electrode for a nonaqueous secondary batteryaccording to claim 1, wherein the active material particles containsilicon and have an average particle size of 0.1 to 10 μm in terms ofD₅₀ and an oxygen concentration of less than 2.5% by weight, theoutermost surface of the active material particles having a siliconconcentration of higher than half of an oxygen concentration of theoutermost surface.
 16. The negative electrode for a nonaqueous secondarybattery according to claim 2, wherein the surface coating layer containsat least one element selected.from the group consisting of Cu, Ag, Ni,Co, Cr, Fe, and In.
 17. The negative electrode for a nonaqueoussecondary battery according to claim 2, wherein the surface coatinglayer is formed by electroplating.
 18. The negative electrode for anonaqueous secondary battery according to claim 2, wherein the surfacecoating layer is formed by sputtering, chemical vapor deposition orphysical vapor deposition.
 19. The negative electrode for a nonaqueoussecondary battery according to claim 2, wherein the surface coatinglayer is formed by rolling an electro-conductive foil.
 20. The negativeelectrode for a nonaqueous secondary battery according to claim 19,wherein the electro-conductive foil is a metal foil or anelectro-conductive plastic foil.
 21. The negative electrode for anonaqueous secondary battery according to claim 2, wherein the activematerial layer is formed by applying a slurry containing the activematerial particles to a surface of the current collector.
 22. Thenegative electrode for a nonaqueous secondary battery according to claim2, wherein the surface coating layer has a thickness of 0.3 to 50 μm,and the active material layer has a thickness of 1 to 100 μm.
 23. Thenegative electrode for a nonaqueous secondary battery according to claim2, wherein the surface coating layer has a thickness of 0.3 to 50 μm,and the active material structure has a thickness of 2 to 100 μm. 24.The negative electrode for a nonaqueous secondary battery according toclaim 2, wherein the surface coating layer has a thickness of 0.3 to 50μm, and the electrode has a total thickness of 2 to 200 μm.
 25. Thenegative electrode for a nonaqueous secondary battery according to claim1, wherein the current collector has a large number of micropores of0.01 to 200 μm in diameter at a density of 5 to 10000 pores per cm² andhas a thickness of 1 to 100 μm.
 26. The negative electrode for anonaqueous secondary battery according to claim 1, wherein the currentcollector is formed of punching metal or expanded metal, each having alarge number of openings each having an opening area of 0.0001 to 4 mm²or metal foam.
 27. The negative electrode for a nonaqueous secondarybattery according to claim 1, wherein the current collector is formed ofelectrolytic metal foil.
 28. A process of producing the negativeelectrode for a nonaqueous secondary battery of claim 4, which comprisesapplying a slurry comprising the active material particles, theelectro-conductive carbon material, a binder, and a diluting solvent toa surface of the current collector, drying the coating to form theactive material layer, and electroplating the active material layer withthe electro-conductive material having low capability of forming alithium compound to form the surface coating layer.
 29. A process ofproducing the negative electrode for a nonaqueous secondary battery ofclaim 4, which comprises applying a slurry comprising the activematerial particles, the electro-conductive carbon material, a binder,and a diluting solvent to a surface of the current collector, drying thecoating to form the active material layer, and depositing theelectro-conductive material having low capability of forming a lithiumcompound on the active material layer by sputtering, chemical vapordeposition or physical vapor deposition to form the surface coatinglayer.
 30. A process of producing the negative electrode for anonaqueous secondary battery of claim 25, which comprises forming a coatof a material different from the material making up the currentcollector on a carrier foil to a thickness of 0.001 to 1 μm,electroplating the carrier foil having the coat with the material makingup the current collector to form the current collector, applying aslurry comprising the active material particles, the electro-conductivecarbon material, a binder, and a diluting solvent to a surface of thecurrent collector, drying the coating to form the active material layer,electroplating the active material layer with the electro-conductivematerial having low capability of forming a lithium compound to form thesurface coating layer, and separating the current collector from thecarrier foil.
 31. A nonaqueous secondary battery having the negativeelectrode for a nonaqueous secondary battery according to claim 1.